One of my client is not filed GSTR1 for the year 2017-18 in the stipulated time. Because of this reason the customer as not got the input credit in GSTR2A. Hence He had deducted the tax part alone from the customer payment in 2019-20. Is it tenable? . For this reason if the customer raises credit note against my client which can be reversed in clients GST payment in the current year or not. Please advice.