adjustment of bill

possible but nw u have to charge gst

customer will give u debit note  as per applicable amount with gst 

u will give him credit note with gst 

ur party will pass entry for return goods

ur company a/c    xxxxx (full amount)

              To  Purchase          xxxxx (amoutnt munus gst )

              To  gst                    xxxx (as per applicable )

U will give him credit for this 

  sale               xxxxx (amoutnt munus gst )

   gst                    xxxx (as per applicable )

          To party     xxxxx (full amount )