Sales return treatment in gst

In either case, as GST returns already filed for the month, credit note can be filed (within time limit mentioned in Sec 34) to reverse the taxes and taxable value. Disclose in GSTR 1 (for regd. CN - CDNR sheet, for unregd. CN - Net off in B2C).

Whether payment is made or not would not have an impact, as one it is cancelled amount would have to be returned. 

Return cannot be revised. Can adjust is subsequent month returns based on Circular 26/2017, upto September of subsequent year (for such FY)

Time limit for raising CN-30th September of subsequent FY. (Ex: for FY 2019-20 - 30th September 2020 last date to raise CN with GST). Although, Commerical CN (without GST) can be raised.