fix: Concurrency issues in Sales and Purchase returns (#34019)

fix: Concurrency issues in Sales and Purchase returns (#34019)

(cherry picked from commit a67284e96d)

Co-authored-by: Deepesh Garg <deepeshgarg6@gmail.com>

erpnext/controllers/sales_and_purchase_return.py

@@ -252,6 +252,7 @@ def get_already_returned_items(doc):
 			child.parent = par.name and par.docstatus = 1
 			and par.is_return = 1 and par.return_against = %s
 		group by item_code
+		for update
 	""".format(
 			column, doc.doctype, doc.doctype
 		),