advance receive

If supplier received a advance for buyer a b2c transaction in the month of July saying 50k
gst on goods to be supply is 5%

And den in the month of Sep 2017 he took goods of 30k. Invoice value 31. 5k
and asked for rest advance amount back.

The supplier have filed gstr 1 of July and showed 50 as advance with gst

How to adjust the gst paid on advance if he give the rest advance amount back