My auditor fas forgot to take some bills on Mar 2019 in GSTR 1 But in GSTR 3B He entered the correct value, may I get the solution for that and what will i do while Filling GSTR 9
You have to show the same in GSTR 9 to make reconcile with book sales as you said he has shown correct figure in 3B that mens you have paid correct amount of GST on sales so you no need to pay any extra. GSTR 9 means if you did any mistakes while filing monthly or quarterly returns you can rectify the same in annual return. Hope it helps.
If you did not file GSTR 1 for March month then you can file now and your customer will get credit in his GSTR 2A. But there is no use because time limit for utilisation of credit for the month of march invoices has already been lapsed by September 2019 itself.
I think your customer might have utilised ITC based on his invoice so better upload now in future he can be safe because your upload invoice will be credited to his GSTR 2A.