diff --git a/README.md b/README.md
index b5b1a91..1398b99 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,7 @@
 - [Pip Installation](#pip-installation)
 - [Example](#example)
 - [iDIN](#idin)
+- [Instant Refunds](#instant-refunds)
 - [Contribute](#contribute)
 - [Versioning](#versioning)
 - [Additional information](#additional-information)
@@ -113,6 +114,24 @@ print("redirect:", response.get_redirect_url())
 
 See [`examples/idin.py`](examples/idin.py) for a runnable demo of all three actions.
 
+### Instant Refunds
+
+Instant refunds send money back to the shopper immediately instead of via the regular batch refund process. They are processed as an instant payment rather than a standard refund, and are supported for iDEAL and Payconiq via `instantRefund()`. Pass the `original_transaction_key` of a settled payment; `refund_amount` is optional — omit it for a full refund.
+
+```python
+response = payments.create_payment("ideal", {
+    "currency": "EUR",
+    "description": "ideal instant refund demo",
+    "invoice": "IDEAL-REFUND-DEMO-001",
+    "original_transaction_key": "ORIGINAL-TRANSACTION-KEY",
+    "refund_amount": 12.34,  # optional; omit for a full refund
+}).instantRefund()
+
+print("key:", response.key)
+```
+
+See [`examples/instant_refund.py`](examples/instant_refund.py) for a runnable demo covering both iDEAL and Payconiq.
+
 ### Contribute
 
 We really appreciate it when developers contribute to improve the Buckaroo plugins.
diff --git a/buckaroo/builders/base_builder.py b/buckaroo/builders/base_builder.py
index 704cb1f..d586d8f 100644
--- a/buckaroo/builders/base_builder.py
+++ b/buckaroo/builders/base_builder.py
@@ -398,15 +398,20 @@ def pay(self, validate: bool = True, strict_validation: bool = False) -> Payment
 
         return self._post_transaction(request_data)
 
-    def refund(self, validate: bool = True) -> PaymentResponse:
+    def _build_refund_request_data(self, action: str, validate: bool = True) -> Dict[str, Any]:
         """
-        Execute a refund transaction.
+        Build the wire request body shared by refund-style actions.
+
+        Reads ``original_transaction_key`` and ``refund_amount`` from the payload,
+        builds the request for ``action``, and swaps ``AmountDebit`` for
+        ``AmountCredit`` (partial or full).
 
         Args:
+            action (str): The Buckaroo action to build (e.g. "Refund", "instantRefund")
             validate (bool): Whether to validate service parameters before building
 
         Returns:
-            PaymentResponse: The refund response
+            Dict[str, Any]: The refund request body
 
         Raises:
             ValueError: If required fields are missing
@@ -422,7 +427,7 @@ def refund(self, validate: bool = True) -> PaymentResponse:
         refund_amount = self._payload.get("refund_amount")
 
         # Build refund request with original transaction reference
-        payment_request = self.build("Refund", validate=validate)
+        payment_request = self.build(action, validate=validate)
 
         # Convert to dictionary and modify for refund
         request_data = payment_request.to_dict()
@@ -438,6 +443,22 @@ def refund(self, validate: bool = True) -> PaymentResponse:
             request_data["AmountCredit"] = request_data["AmountDebit"]
             del request_data["AmountDebit"]
 
+        return request_data
+
+    def refund(self, validate: bool = True) -> PaymentResponse:
+        """
+        Execute a refund transaction.
+
+        Args:
+            validate (bool): Whether to validate service parameters before building
+
+        Returns:
+            PaymentResponse: The refund response
+
+        Raises:
+            ValueError: If required fields are missing
+        """
+        request_data = self._build_refund_request_data("Refund", validate)
         return self._post_transaction(request_data)
 
     def pay_remainder(
diff --git a/buckaroo/builders/payments/capabilities/instant_refund_capable.py b/buckaroo/builders/payments/capabilities/instant_refund_capable.py
index 019f78a..2a9a577 100644
--- a/buckaroo/builders/payments/capabilities/instant_refund_capable.py
+++ b/buckaroo/builders/payments/capabilities/instant_refund_capable.py
@@ -7,7 +7,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 from ....models.payment_response import PaymentResponse
 
@@ -18,19 +18,48 @@
 class InstantRefundCapable:
     """Mixin for payment methods that support instant refunds (iDEAL, Sofort, PayConiq)."""
 
-    def instantRefund(self: "PaymentBuilder", validate: bool = True) -> PaymentResponse:
+    def instantRefund(
+        self: "PaymentBuilder",
+        original_transaction_key: Optional[str] = None,
+        validate: bool = True,
+    ) -> PaymentResponse:
         """
         Initiate an instant refund.
 
+        Mirrors :meth:`BaseBuilder.refund`: puts ``OriginalTransactionKey`` and
+        ``AmountCredit`` on the wire instead of ``AmountDebit``, but keeps the
+        ``instantRefund`` action name.
+
         Available for: iDEAL, Sofort, PayConiq
         Not available for: Credit Card, PayPal (use regular refund instead)
 
         Args:
+            original_transaction_key (str, optional): The transaction key of the
+                original payment. If None, will try to get from payload.
             validate (bool): Whether to validate service parameters before building
 
         Returns:
             PaymentResponse: The instant refund response
+
+        Raises:
+            ValueError: If no original transaction key is available
         """
-        payment_request = self.build("instantRefund", validate=validate)
-        request_data = payment_request.to_dict()
+        txn_key = original_transaction_key or self._payload.get("original_transaction_key")
+        if not txn_key:
+            raise ValueError(
+                "Original transaction key is required for instant refunds "
+                "(provide as parameter or in payload)"
+            )
+
+        _MISSING = object()
+        prev_key = self._payload.get("original_transaction_key", _MISSING)
+        self._payload["original_transaction_key"] = txn_key
+        try:
+            request_data = self._build_refund_request_data("instantRefund", validate)
+        finally:
+            if prev_key is _MISSING:
+                self._payload.pop("original_transaction_key", None)
+            else:
+                self._payload["original_transaction_key"] = prev_key
+
         return self._post_transaction(request_data)
diff --git a/examples/instant_refund.py b/examples/instant_refund.py
new file mode 100644
index 0000000..4827b69
--- /dev/null
+++ b/examples/instant_refund.py
@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""Demo of instant refunds for iDEAL and Payconiq.
+
+Instant refunds send money back to the shopper immediately instead of via the
+regular batch refund process. They are supported for iDEAL and Payconiq
+through ``instantRefund()``, which reads ``original_transaction_key`` from the
+payload (or an optional argument) and an optional ``refund_amount`` for a
+partial refund (a full refund is issued when it's omitted). The refund is
+processed as an instant payment on the wire: ``OriginalTransactionKey`` plus
+``AmountCredit`` instead of ``AmountDebit``. Gated on ``BUCKAROO_STORE_KEY`` /
+``BUCKAROO_SECRET_KEY`` env vars.
+"""
+
+import os
+import sys
+
+# Add parent directory to Python path so the demo can import the SDK in-place.
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+from buckaroo.app import Buckaroo
+
+# Placeholder transaction keys. Replace with the key of a SETTLED iDEAL /
+# Payconiq payment before running for real; a live green instant refund also
+# requires a merchant account with instant refunds enabled.
+_IDEAL_ORIGINAL_TRANSACTION_KEY = "REPLACE_WITH_SETTLED_IDEAL_TRANSACTION_KEY"
+_PAYCONIQ_ORIGINAL_TRANSACTION_KEY = "REPLACE_WITH_SETTLED_PAYCONIQ_TRANSACTION_KEY"
+
+
+def _have_credentials() -> bool:
+    if not os.getenv("BUCKAROO_STORE_KEY") or not os.getenv("BUCKAROO_SECRET_KEY"):
+        print("⚠️  Set BUCKAROO_STORE_KEY and BUCKAROO_SECRET_KEY to run this demo")
+        return False
+    return True
+
+
+def demo_ideal_instant_refund() -> None:
+    """Instantly refund part of a settled iDEAL payment."""
+    print("\n1. iDEAL instant refund")
+    print("-" * 40)
+    if not _have_credentials():
+        return
+
+    try:
+        app = Buckaroo.from_env()
+        response = app.payments.create_payment(
+            "ideal",
+            {
+                "currency": "EUR",
+                "description": "ideal instant refund demo",
+                "invoice": "IDEAL-REFUND-DEMO-001",
+                "original_transaction_key": _IDEAL_ORIGINAL_TRANSACTION_KEY,
+                "refund_amount": 12.34,  # optional; omit for full refund
+            },
+        ).instantRefund()
+        print(f"   status.code={response.status.code.code}  key={response.key}")
+    except Exception as e:
+        print(f"   ❌ {e}")
+
+
+def demo_payconiq_instant_refund() -> None:
+    """Instantly refund part of a settled Payconiq payment."""
+    print("\n2. Payconiq instant refund")
+    print("-" * 40)
+    if not _have_credentials():
+        return
+
+    try:
+        app = Buckaroo.from_env()
+        response = app.payments.create_payment(
+            "payconiq",
+            {
+                "currency": "EUR",
+                "description": "payconiq instant refund demo",
+                "invoice": "PAYCONIQ-REFUND-DEMO-001",
+                "original_transaction_key": _PAYCONIQ_ORIGINAL_TRANSACTION_KEY,
+                "refund_amount": 12.34,  # optional; omit for full refund
+            },
+        ).instantRefund()
+        print(f"   status.code={response.status.code.code}  key={response.key}")
+    except Exception as e:
+        print(f"   ❌ {e}")
+
+
+def main() -> None:
+    print("BUCKAROO SDK — INSTANT REFUND DEMO")
+    print("=" * 60)
+    demo_ideal_instant_refund()
+    demo_payconiq_instant_refund()
+    print("\n" + "=" * 60)
+    print("done.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/feature/payments/test_ideal.py b/tests/feature/payments/test_ideal.py
index 01c1be9..625424c 100644
--- a/tests/feature/payments/test_ideal.py
+++ b/tests/feature/payments/test_ideal.py
@@ -1,5 +1,7 @@
+import pytest
+
 from tests.support.mock_request import BuckarooMockRequest
-from tests.support.recording_mock import recorded_action
+from tests.support.recording_mock import recorded_action, recorded_request
 from tests.support.helpers import Helpers
 
 
@@ -86,6 +88,18 @@ def test_ideal_instant_refund_sends_action_instantrefund_on_the_wire(
         ).instantRefund()
 
         assert recorded_action(recording_mock) == "instantRefund"
+        request = recorded_request(recording_mock)
+        assert request["OriginalTransactionKey"] == "ABC123"
+        assert request["AmountCredit"] == 10.00
+        assert "AmountDebit" not in request
+
+    def test_ideal_instant_refund_raises_without_original_transaction_key(self, recording_buckaroo):
+        """A missing ``original_transaction_key`` must fail fast, matching ``refund()``."""
+        with pytest.raises(ValueError, match="Original transaction key is required"):
+            recording_buckaroo.payments.create_payment(
+                "ideal",
+                Helpers.standard_payload(invoice="INV-IDEAL-WIRE-IREFUND-NOKEY"),
+            ).instantRefund()
 
     def test_ideal_fast_checkout_sends_action_payfastcheckout_on_the_wire(
         self, recording_buckaroo, recording_mock
diff --git a/tests/feature/payments/test_payconiq.py b/tests/feature/payments/test_payconiq.py
index e22ff81..77ef04b 100644
--- a/tests/feature/payments/test_payconiq.py
+++ b/tests/feature/payments/test_payconiq.py
@@ -1,5 +1,9 @@
 """Feature test: payconiq pay() and capability methods through full stack with MockBuckaroo."""
 
+import pytest
+
+from tests.support.mock_request import BuckarooMockRequest
+from tests.support.recording_mock import recorded_action, recorded_request
 from tests.support.helpers import Helpers
 
 
@@ -38,3 +42,114 @@ def test_payconiq_fast_checkout(self, buckaroo, mock_strategy):
             method="payconiq",
             invoice="INV-PCQ-FAST",
         )
+
+    # ------------------------------------------------------------------
+    # Wire-level assertions — verify BankTransferCapabilities' InstantRefund
+    # mixin puts the right Action, OriginalTransactionKey and AmountCredit
+    # on the outgoing request for Payconiq (partial + full).
+
+    def test_payconiq_instant_refund_full_sends_action_instantrefund_on_the_wire(
+        self, recording_buckaroo, recording_mock
+    ):
+        """Full refund (``refund_amount`` omitted): AmountDebit swaps to AmountCredit."""
+        recording_mock.queue(
+            BuckarooMockRequest.json(
+                "POST",
+                "*/json/transaction*",
+                Helpers.success_response(
+                    {
+                        "Services": [
+                            {"Name": "payconiq", "Action": "InstantRefund", "Parameters": []}
+                        ],
+                        "ServiceCode": "payconiq",
+                        "AmountCredit": 10.00,
+                        "AmountDebit": None,
+                    }
+                ),
+            )
+        )
+        recording_buckaroo.payments.create_payment(
+            "payconiq",
+            Helpers.standard_payload(
+                invoice="INV-PCQ-WIRE-IREFUND-FULL",
+                original_transaction_key="ABC123",
+            ),
+        ).instantRefund()
+
+        assert recorded_action(recording_mock) == "instantRefund"
+        request = recorded_request(recording_mock)
+        assert request["Services"]["ServiceList"][0]["Name"] == "payconiq"
+        assert request["OriginalTransactionKey"] == "ABC123"
+        assert request["AmountCredit"] == 10.00
+        assert "AmountDebit" not in request
+
+    def test_payconiq_instant_refund_partial_sends_refund_amount_as_credit(
+        self, recording_buckaroo, recording_mock
+    ):
+        """Partial refund (``refund_amount`` in payload): AmountCredit is that amount."""
+        recording_mock.queue(
+            BuckarooMockRequest.json(
+                "POST",
+                "*/json/transaction*",
+                Helpers.success_response(
+                    {
+                        "Services": [
+                            {"Name": "payconiq", "Action": "InstantRefund", "Parameters": []}
+                        ],
+                        "ServiceCode": "payconiq",
+                        "AmountCredit": 4.00,
+                        "AmountDebit": None,
+                    }
+                ),
+            )
+        )
+        recording_buckaroo.payments.create_payment(
+            "payconiq",
+            Helpers.standard_payload(
+                invoice="INV-PCQ-WIRE-IREFUND-PARTIAL",
+                original_transaction_key="ABC123",
+                refund_amount=4.00,
+            ),
+        ).instantRefund()
+
+        assert recorded_action(recording_mock) == "instantRefund"
+        request = recorded_request(recording_mock)
+        assert request["Services"]["ServiceList"][0]["Name"] == "payconiq"
+        assert request["OriginalTransactionKey"] == "ABC123"
+        assert request["AmountCredit"] == 4.00
+        assert "AmountDebit" not in request
+
+    def test_payconiq_instant_refund_raises_without_original_transaction_key(
+        self, recording_buckaroo
+    ):
+        """A missing ``original_transaction_key`` must fail fast, matching iDEAL."""
+        with pytest.raises(ValueError, match="Original transaction key is required"):
+            recording_buckaroo.payments.create_payment(
+                "payconiq",
+                Helpers.standard_payload(invoice="INV-PCQ-WIRE-IREFUND-NOKEY"),
+            ).instantRefund()
+
+    def test_payconiq_instant_refund_surfaces_api_rejection_as_failed_response(
+        self, buckaroo, mock_strategy
+    ):
+        """A rejected instant refund is surfaced as a standard failed response,
+        not an exception."""
+        response_body = Helpers.failed_response(
+            "Refund rejected",
+            overrides={
+                "Services": [{"Name": "payconiq", "Action": "InstantRefund", "Parameters": []}],
+                "ServiceCode": "payconiq",
+                "AmountCredit": None,
+                "AmountDebit": 10.00,
+            },
+        )
+        mock_strategy.queue(BuckarooMockRequest.json("POST", "*/json/transaction", response_body))
+        payload = Helpers.standard_payload(
+            invoice="INV-PCQ-IREFUND-REJECTED",
+            original_transaction_key="ABC123",
+        )
+        response = buckaroo.payments.create_payment("payconiq", payload).instantRefund()
+
+        assert response.is_failed()
+        assert not response.is_successful()
+        assert response.key == response_body["Key"]
diff --git a/tests/support/helpers.py b/tests/support/helpers.py
index 2a5dc5f..2ba10d3 100644
--- a/tests/support/helpers.py
+++ b/tests/support/helpers.py
@@ -235,6 +235,10 @@ def assert_instant_refund_returns_success(
         assert response.status.code.code == STATUS_SUCCESS
         assert response.key == response_body["Key"]
         _assert_recorded_action(mock_strategy, "instantRefund")
+        sent = json.loads(mock_strategy.calls[-1]["data"])
+        assert sent["OriginalTransactionKey"] == original_transaction_key
+        assert sent["AmountCredit"] == 10.00
+        assert "AmountDebit" not in sent
         return response
 
     @staticmethod
diff --git a/tests/unit/builders/payments/capabilities/test_bank_transfer_capabilities.py b/tests/unit/builders/payments/capabilities/test_bank_transfer_capabilities.py
index 25107e6..265aca3 100644
--- a/tests/unit/builders/payments/capabilities/test_bank_transfer_capabilities.py
+++ b/tests/unit/builders/payments/capabilities/test_bank_transfer_capabilities.py
@@ -10,6 +10,8 @@
 
 from __future__ import annotations
 
+import pytest
+
 from buckaroo.builders.payments.capabilities.bank_transfer_capabilities import (
     BankTransferCapabilities,
 )
@@ -22,7 +24,7 @@
 from buckaroo.models.payment_response import PaymentResponse
 from tests.support.builders import make_test_builder, populate_required_fields
 from tests.support.mock_request import BuckarooMockRequest
-from tests.support.recording_mock import recorded_action, wire_recording_http
+from tests.support.recording_mock import recorded_action, recorded_request, wire_recording_http
 
 
 # ---------------------------------------------------------------------------
@@ -69,7 +71,7 @@ def test_posts_action_instantRefund(self):
         mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
         builder = _ready_builder(client)
 
-        builder.instantRefund(validate=False)
+        builder.instantRefund("ABC123", validate=False)
 
         assert recorded_action(mock) == "instantRefund"
 
@@ -78,7 +80,7 @@ def test_posts_to_transaction_endpoint(self):
         mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
         builder = _ready_builder(client)
 
-        builder.instantRefund(validate=False)
+        builder.instantRefund("ABC123", validate=False)
 
         assert len(mock.calls) == 1
         call = mock.calls[0]
@@ -96,11 +98,30 @@ def test_returns_payment_response(self):
         )
         builder = _ready_builder(client)
 
-        response = builder.instantRefund(validate=False)
+        response = builder.instantRefund("ABC123", validate=False)
 
         assert isinstance(response, PaymentResponse)
         assert response.key == "refund-123"
 
+    def test_sends_original_transaction_key_and_amount_credit_on_the_wire(self):
+        mock, client = wire_recording_http()
+        mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
+        builder = _ready_builder(client)
+
+        builder.instantRefund("ABC123", validate=False)
+
+        request = recorded_request(mock)
+        assert request["OriginalTransactionKey"] == "ABC123"
+        assert request["AmountCredit"] == 10.0
+        assert "AmountDebit" not in request
+
+    def test_raises_value_error_without_original_transaction_key(self):
+        _mock, client = wire_recording_http()
+        builder = _ready_builder(client)
+
+        with pytest.raises(ValueError, match="Original transaction key is required"):
+            builder.instantRefund(validate=False)
+
 
 # ---------------------------------------------------------------------------
 # payFastCheckout()
diff --git a/tests/unit/builders/payments/capabilities/test_instant_refund_capable.py b/tests/unit/builders/payments/capabilities/test_instant_refund_capable.py
index b024c52..e27be32 100644
--- a/tests/unit/builders/payments/capabilities/test_instant_refund_capable.py
+++ b/tests/unit/builders/payments/capabilities/test_instant_refund_capable.py
@@ -52,7 +52,7 @@ def test_posts_action_instantRefund(self):
         mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
         builder = _ready_builder(client)
 
-        builder.instantRefund(validate=False)
+        builder.instantRefund("ABC123", validate=False)
 
         assert recorded_action(mock) == "instantRefund"
 
@@ -61,7 +61,7 @@ def test_posts_to_transaction_endpoint(self):
         mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
         builder = _ready_builder(client)
 
-        builder.instantRefund(validate=False)
+        builder.instantRefund("ABC123", validate=False)
 
         assert len(mock.calls) == 1
         call = mock.calls[0]
@@ -73,7 +73,7 @@ def test_posts_expected_service_name(self):
         mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
         builder = _ready_builder(client)
 
-        builder.instantRefund(validate=False)
+        builder.instantRefund("ABC123", validate=False)
 
         service = recorded_request(mock)["Services"]["ServiceList"][0]
         assert service["Name"] == "ideal"
@@ -89,11 +89,54 @@ def test_returns_payment_response(self):
         )
         builder = _ready_builder(client)
 
-        response = builder.instantRefund(validate=False)
+        response = builder.instantRefund("ABC123", validate=False)
 
         assert isinstance(response, PaymentResponse)
         assert response.key == "refund-123"
 
+    def test_raises_value_error_without_original_transaction_key(self):
+        _mock, client = wire_recording_http()
+        builder = _ready_builder(client)
+
+        with pytest.raises(ValueError, match="Original transaction key is required"):
+            builder.instantRefund(validate=False)
+
+    def test_full_refund_sends_original_transaction_key_and_swaps_amount_to_credit(self):
+        mock, client = wire_recording_http()
+        mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
+        builder = _ready_builder(client)
+
+        builder.instantRefund("ABC123", validate=False)
+
+        request = recorded_request(mock)
+        assert request["OriginalTransactionKey"] == "ABC123"
+        assert request["AmountCredit"] == 10.0
+        assert "AmountDebit" not in request
+
+    def test_partial_refund_uses_refund_amount_from_payload(self):
+        mock, client = wire_recording_http()
+        mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
+        builder = _ready_builder(client)
+        builder._payload["refund_amount"] = 3.25
+
+        builder.instantRefund("ABC123", validate=False)
+
+        request = recorded_request(mock)
+        assert request["OriginalTransactionKey"] == "ABC123"
+        assert request["AmountCredit"] == 3.25
+        assert "AmountDebit" not in request
+
+    def test_reads_original_transaction_key_from_payload_fallback(self):
+        mock, client = wire_recording_http()
+        mock.queue(BuckarooMockRequest.json("POST", "*/json/transaction*", {"Key": "ok"}))
+        builder = _ready_builder(client)
+        builder._payload["original_transaction_key"] = "FROM-PAYLOAD"
+
+        builder.instantRefund(validate=False)
+
+        request = recorded_request(mock)
+        assert request["OriginalTransactionKey"] == "FROM-PAYLOAD"
+
     def test_forwards_validate_flag_true_triggers_validation(self):
         """``validate=True`` runs the real service-parameter validator against
         ``get_allowed_service_parameters("instantRefund")`` — so declaring a
@@ -111,6 +154,6 @@ def test_forwards_validate_flag_true_triggers_validation(self):
         )
 
         with pytest.raises(RequiredParameterMissingError) as exc:
-            builder.instantRefund(validate=True)
+            builder.instantRefund("ABC123", validate=True)
 
         assert exc.value.parameter_name == "refund_reason"
diff --git a/tests/unit/builders/payments/test_payconiq_builder.py b/tests/unit/builders/payments/test_payconiq_builder.py
index 53ceb8c..e0a2152 100644
--- a/tests/unit/builders/payments/test_payconiq_builder.py
+++ b/tests/unit/builders/payments/test_payconiq_builder.py
@@ -15,6 +15,7 @@
 )
 from tests.support.mock_request import BuckarooMockRequest
 from tests.support.builders import populate_required_fields
+from tests.support.recording_mock import recorded_action, recorded_request
 
 
 def test_construction_with_client_succeeds(client):
@@ -135,10 +136,38 @@ def test_payconiq_instantRefund_works(client, mock_strategy):
         )
     )
     builder = populate_required_fields(PayconiqBuilder(client))
-    response = builder.instantRefund(validate=False)
+    response = builder.instantRefund("ABC123", validate=False)
     assert response is not None
 
 
+def test_payconiq_instantRefund_sends_original_transaction_key_and_amount_credit_on_wire(
+    client, mock_strategy
+):
+    """InstantRefundCapable (inherited via BankTransferCapabilities) must put
+    OriginalTransactionKey + AmountCredit on the wire for Payconiq, action instantRefund."""
+    mock_strategy.queue(
+        BuckarooMockRequest.json(
+            "POST", "*/json/transaction*", {"Key": "pcq-ir-2", "Status": {"Code": {"Code": 190}}}
+        )
+    )
+    builder = populate_required_fields(PayconiqBuilder(client))
+
+    builder.instantRefund("ABC123", validate=False)
+
+    assert recorded_action(mock_strategy) == "instantRefund"
+    request = recorded_request(mock_strategy)
+    assert request["OriginalTransactionKey"] == "ABC123"
+    assert request["AmountCredit"] == 10.0
+    assert "AmountDebit" not in request
+
+
+def test_payconiq_instantRefund_raises_without_original_transaction_key(client, mock_strategy):
+    builder = populate_required_fields(PayconiqBuilder(client))
+
+    with pytest.raises(ValueError, match="Original transaction key is required"):
+        builder.instantRefund(validate=False)
+
+
 def test_pay_end_to_end(client, mock_strategy):
     mock_strategy.queue(
         BuckarooMockRequest.json(
diff --git a/tests/unit/builders/payments/test_sofort_builder.py b/tests/unit/builders/payments/test_sofort_builder.py
index 51b7da5..962248f 100644
--- a/tests/unit/builders/payments/test_sofort_builder.py
+++ b/tests/unit/builders/payments/test_sofort_builder.py
@@ -144,7 +144,7 @@ def test_instant_refund_works(client, mock_strategy):
         )
     )
     builder = populate_required_fields(SofortBuilder(client))
-    response = builder.instantRefund()
+    response = builder.instantRefund("ABC123")
     assert response is not None