Adding missing details 2017_18

Venkatesan Ramanujam (123 Points)

09 December 2019  
we have filed annual return for the year 2017-18. we have filed our monthly return GSTR1 for the month of July 2017. But from the month of August we opted to file in quarterly basis. But while filing September quarterly return we have forgotten to add details ie. sales of August.
since we opted for quarterly return the system allowed us to file annual return with the sales details of August.
Since we have not added the details of August the buyers were not able to make use of their input tax they availed by them. And they also asked to repay the Tax paid by them. Please guide me how to add missing details and avoid losses.