Ur buyer will upload their invoice in GSTR2 as missing invoice and it will be reflected in ur GSTR1A... Then accept it... And amend b2c details in subsequent months gstr1 in amended b2c column
Be careful while filling ur GST return... This type of mistake is not silly mistakes... Plz make correct entries of ur all invoices in ur software whatever used either Tally or something else
If u miss ur b2b transaction in gstr1 then ur buyer will upload it as missing invoice in their gstr2 and then reflecting in ur GSTR1A.... Once all gstr2 has been filed only then gstr3 final return has been filed.... U hv the option either amend it in next month gstr1 or ur buyer will upload it when gstr2 filing is enabled.... So u can't escape
He already filed Gstr1 for two months so he can't able to amend gstr1 of 2 months earlier... In GSTR1 only amendment is possible for previous month.... U hv the option what u choose... But u can't escape adding ur b2b transaction at any level... Ur buyer will also hv the option to add the invoice
she can amend 2 month gstr 1 there is no prob ... B2CS basis on monthly amend you can choos which month you want to amend B2B amend on invoice number basis