for composition scheme

The supplier under composition scheme is required file GSTR 4 (QUARTERLY) AND GSTR 9A (Annual) return.

Dear Sir,

A business was registed under GST as regular taxpayer for july and agust but in sep, he has opted composition scheme as turnover wud be less than 75 lacs. He has already filed GSTR 1 for July and GSTR 3b for August and paid the tax dues. Please confirm how can he take credit of the tax paid by him. Alos when we login to gst portal, we cant see the returns filed by him and only open window comes for July-sep. So, does he has to file a quarterly return for July to sep again? what will happen to return already filed and tax already paid