fix: system was allowing credit notes with serial numbers for any customer

erpnext/controllers/sales_and_purchase_return.py

@@ -38,6 +38,17 @@ def validate_return_against(doc):
 
 		party_type = "customer" if doc.doctype in ("Sales Invoice", "Delivery Note") else "supplier"
 
+		if ref_doc.get(party_type) != doc.get(party_type):
+			frappe.throw(
+				_("The {0} {1} does not match with the {0} {2} in the {3} {4}").format(
+					doc.meta.get_label(party_type),
+					doc.get(party_type),
+					ref_doc.get(party_type),
+					ref_doc.doctype,
+					ref_doc.name,
+				)
+			)
+
 		if (
 			ref_doc.company == doc.company
 			and ref_doc.get(party_type) == doc.get(party_type)

erpnext/controllers/selling_controller.py

@@ -61,6 +61,35 @@ class SellingController(StockController):
 			if self.get(table_field):
 				self.set_serial_and_batch_bundle(table_field)
 
+	def validate_standalone_serial_nos_customer(self):
+		if not self.is_return or self.return_against:
+			return
+
+		if self.doctype in ["Sales Invoice", "Delivery Note"]:
+			bundle_ids = [d.serial_and_batch_bundle for d in self.get("items") if d.serial_and_batch_bundle]
+			if not bundle_ids:
+				return
+
+			serial_nos = frappe.get_all(
+				"Serial and Batch Entry",
+				filters={"parent": ("in", bundle_ids)},
+				pluck="serial_no",
+			)
+
+			if serial_nos := frappe.get_all(
+				"Serial No",
+				filters={"name": ("in", serial_nos), "customer": ("is", "set")},
+				fields=["name", "customer"],
+			):
+				for sn in serial_nos:
+					if sn.customer and sn.customer != self.customer:
+						frappe.throw(
+							_(
+								"Serial No {0} is already assigned to customer {1}. Can only be returned against the customer {1}"
+							).format(frappe.bold(sn.name), frappe.bold(sn.customer)),
+							title=_("Serial No Already Assigned"),
+						)
+
 	def set_missing_values(self, for_validate=False):
 		super().set_missing_values(for_validate)