Sale Mismatch in GSTR1 & 3B

If there are differences between GSTR-1 and GSTR-3B, the same needs to be rectified in next month return not later than September return of next FY.

since you are telling the difference is in 2018-19, rectification can be done in Aug'19 or Sep'19 return

As rightly mentioned by Sujith, pls amend your invoices in the subsequent GSTR-1 and try to reconcile both. This would help in annual return of that period as well.

If there are mismatch in R1 then you have to amend them in next pending GSTR-1 but 1 thing you have to mind it if there are differences in B2C then you have to do amendments in table 10 many of the peoples had arisen the figures in table 7 so it's a thing to mind it.

if there are differences in GSTR-3B then you have add less the reconciled figures in next pending return.

and these changes amendments can be done before Sep 19 it's not sure the date will be extended or not to do amendments and there are changes in return filling system so it can't be said in those returns it's possible to amend the olds or not