GSTR 1

As per my observation,there will be 2 parts of gstr3, partA and partB.....u can modify the figures of sales in partB of gstr3 which is supposed to fill by 10th Nov for the month of july.this gstr3 will be treated as final return for the tax purposes.Alternatively u can tell to the buyer to fill the correct figure of ur sale which is known as his purchase,which the buyer can show in his gstr2 by 31st oct

ones the buyer files his gstr2 with correct figures,it will get reflected in sellers gstr1A,then the seller have to accept them

but buyer is unregistered.. he will not file any GST return

no issues,u can modify it in gstr3 as I told u

thanks