case-1) sales return in jul amtg rs. two Lacs pertains to 4/17-6/17-- file refund claim for tax paid in old regime (if returned by its) otherwise it would be under GST in the hands of your customer (section 142 referred) . for bal sale return amtg to rs. 25000, issue credit note under section 34, i.e prior to sept of next FY . case-2) sales return in jul amtg to rs. two Lacs pertains to jul itself-- issue credit note as per section 34 and for balance sales return same treatment as in case-1. REGARDS. PS: credit/debit notes are shown in GSTR-1.