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

diff --git a/erpnext/controllers/sales_and_purchase_return.py b/erpnext/controllers/sales_and_purchase_return.py
index 9fcb769..fc6793a 100644
--- a/erpnext/controllers/sales_and_purchase_return.py
+++ b/erpnext/controllers/sales_and_purchase_return.py
@@ -252,6 +252,7 @@
 			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
 		),