We have made mistake in putting GSTIN No. in one of our customer , now customer want correct the same. Invoice made in Dec 2017, and as Govt. we have to correct the same in our Sept GSTR1 return , but customer send mesasge to us on 26.11.2018 to rectify the same.
28 November 2018
Yes technically it was allowed till september but if GSTR-1 facility on the portal still allows you to make amendments in B2B section you may go ahead. If your return filing frequency is monthly then you may obviously correct it in the month of october GSTR-1 but if it is quarterly then only after quarter end if portal permits.