fix: use invoice outstanding on Dunning (#41817)

(cherry picked from commit cf0a29b05f)

erpnext/accounts/doctype/sales_invoice/sales_invoice.py

@@ -2678,6 +2678,10 @@ def create_dunning(source_name, target_doc=None, ignore_permissions=False):
 				target.closing_text = letter_text.get("closing_text")
 				target.language = letter_text.get("language")
 
+		# update outstanding
+		if source.payment_schedule and len(source.payment_schedule) == 1:
+			target.overdue_payments[0].outstanding = source.get("outstanding_amount")
+
 		target.validate()
 
 	return get_mapped_doc(