Gstr4 annual return mistake 1

sir,

A gst registered composite dealer annual return gstr4 filed in f.y.19-20. but dealer mistake sales turnover reported table 6 tax rate wise negative liability show in annual return gstr4 in f.y.20-21.

Question:

1. Dealer previous sales turnover added to outward supplies negative liability set off adjustment f.y.20-21 in gstr4 allowed in gst act.

2.composite dealer annual returns gstr4 and gstr9a both  returns filed or gstr4 only filed in f.y.20-21.