fix: use transaction-currency outstanding on Dunning for foreign-currency invoices

When a Sales Invoice is in a foreign currency (e.g. USD) but the receivable
account is in the company currency (e.g. INR), `outstanding_amount` on the
invoice is stored in the party account currency (INR). `postprocess_dunning`
was copying that value directly into the Dunning's Overdue Payment row, which
is expected to carry the transaction-currency (USD) amount.

The fix: when `party_account_currency != currency`, use
`payment_schedule[0].outstanding` (already maintained in transaction currency)
instead of `outstanding_amount`.

Closes #56006

erpnext/accounts/doctype/dunning/test_dunning.py

@@ -12,6 +12,7 @@ from erpnext.accounts.doctype.sales_invoice.mapper import (
 	create_dunning as create_dunning_from_sales_invoice,
 )
 from erpnext.accounts.doctype.sales_invoice.test_sales_invoice import (
+	create_sales_invoice,
 	create_sales_invoice_against_cost_center,
 )
 from erpnext.tests.utils import ERPNextTestSuite
@@ -152,6 +153,37 @@ class TestDunning(ERPNextTestSuite):
 		dunning.reload()
 		self.assertEqual(dunning.status, "Unresolved")
 
+	@ERPNextTestSuite.change_settings(
+		"Accounts Settings", {"allow_multi_currency_invoices_against_single_party_account": 1}
+	)
+	def test_dunning_outstanding_uses_transaction_currency(self):
+		"""
+		Regression for #56006: dunning outstanding must be in the invoice transaction
+		currency, not in the party account currency.
+
+		A USD invoice posted against an INR receivable account stores
+		outstanding_amount in INR (party account currency).  The overdue payment
+		row on the resulting Dunning must carry the USD amount, not the INR amount.
+		"""
+		si = create_sales_invoice(
+			posting_date=add_days(today(), -10),
+			currency="USD",
+			conversion_rate=50,
+			rate=100,
+			debit_to="Debtors - _TC",
+		)
+
+		# Sanity-check the invoice state before creating the dunning
+		self.assertEqual(si.currency, "USD")
+		self.assertEqual(si.outstanding_amount, 5000.0)  # INR (party account currency)
+		self.assertEqual(si.payment_schedule[0].outstanding, 100.0)  # USD (transaction currency)
+
+		dunning = create_dunning_from_sales_invoice(si.name)
+
+		self.assertEqual(len(dunning.overdue_payments), 1)
+		# Must reflect 100 USD, not 5000 INR mislabelled as USD
+		self.assertEqual(dunning.overdue_payments[0].outstanding, 100.0)
+
 	def test_dunning_not_affected_by_standalone_credit_note(self):
 		"""
 		Test that dunning is NOT resolved when a credit note has update_outstanding_for_self checked.

erpnext/accounts/doctype/sales_invoice/mapper.py

@@ -594,7 +594,15 @@ def create_dunning(
 		if source.payment_schedule and len(source.payment_schedule) == 1:
 			for row in target.overdue_payments:
 				if row.payment_schedule == source.payment_schedule[0].name:
-					row.outstanding = source.get("outstanding_amount")
+					# outstanding_amount is in the party account currency, but the Overdue Payment
+					# row is in the invoice's transaction currency. When they differ, use the
+					# payment schedule's own outstanding — it is kept in transaction currency and
+					# updated as payments are allocated, so it stays correct even when the invoice
+					# and its payments post at different exchange rates (#56006).
+					if source.party_account_currency and source.party_account_currency != source.currency:
+						row.outstanding = source.payment_schedule[0].outstanding
+					else:
+						row.outstanding = source.get("outstanding_amount")
 
 		target.validate()