diff --git a/commerce_coordinator/apps/paypal/clients.py b/commerce_coordinator/apps/paypal/clients.py
index b44680adf..2f2038c26 100644
--- a/commerce_coordinator/apps/paypal/clients.py
+++ b/commerce_coordinator/apps/paypal/clients.py
@@ -32,6 +32,9 @@ def refund_order(self, capture_id, amount):
 
         Args:
             capture_id (str): The identifier of the PayPal order to capture refund.
+            amount (decimal.Decimal | int | float): Refund amount; coerced to str for the
+                request body because the PayPal SDK serializes the body with jsonpickle,
+                which encodes Decimal as null (PayPal expects Money.value as a string).
 
         Returns:
             The response from PayPal.
@@ -45,7 +48,7 @@ def refund_order(self, capture_id, amount):
             "prefer": "return=representation",
             "body": {
                 "amount": {
-                    "value": amount,
+                    "value": str(amount),
                     "currency_code": "USD"
                 }
             }
diff --git a/commerce_coordinator/apps/paypal/tests/test_clients.py b/commerce_coordinator/apps/paypal/tests/test_clients.py
new file mode 100644
index 000000000..0ef5ab73b
--- /dev/null
+++ b/commerce_coordinator/apps/paypal/tests/test_clients.py
@@ -0,0 +1,41 @@
+"""Tests for PayPal API client."""
+
+import json
+from decimal import Decimal
+from unittest.mock import MagicMock, patch
+
+from django.test import TestCase
+
+from commerce_coordinator.apps.paypal.clients import PayPalClient
+
+
+class PayPalClientRefundOrderTests(TestCase):
+    """Regression tests for refund_order request shaping."""
+
+    @patch("commerce_coordinator.apps.paypal.clients.PaypalServersdkClient")
+    @patch("commerce_coordinator.apps.paypal.clients.ApiHelper.json_serialize")
+    def test_refund_order_serializes_decimal_amount_as_string(
+        self, mock_json_serialize, mock_sdk_class
+    ):
+        """
+        PayPal Money.value is a string. The refund amount may come from
+        get_line_item_price_to_refund as a Decimal, and the SDK body uses
+        jsonpickle, which emits null for Decimal unless we pass a string.
+        """
+        mock_json_serialize.return_value = json.dumps(
+            {
+                "id": "REFUND_ID",
+                "create_time": "2024-01-01T00:00:00Z",
+                "status": "COMPLETED",
+                "amount": {"value": "49.00", "currency_code": "USD"},
+            }
+        )
+        mock_payments = MagicMock()
+        mock_sdk_class.return_value.payments = mock_payments
+
+        client = PayPalClient()
+        client.refund_order("CAPTURE_ID", Decimal("49.00"))
+
+        mock_payments.refund_captured_payment.assert_called_once()
+        collect = mock_payments.refund_captured_payment.call_args[0][0]
+        self.assertEqual(collect["body"]["amount"]["value"], "49.00")