Buyer had manually issued me a debit note against the bill of previous month (July 2018) for which we have already filed a returns. Debit note has not found in GSTR 2A. 
1) Can I Issue a Credit Note in GSTR 1 for December 2018?
2) Will the excess GST paid amount will be credited to ITC Ledger?
3) Should I less the credit note amount from current month sale and file the GSTR 3B?

Reply

1) You can show credit note in GSTR-1 of November, 2018 due date of which will be in December month, and the date of such credit note should be according to the date when buyer has reversed its ITC for his purchase return.

2) Excess GST paid amount will not be credited to ITC ledger.

3) While filing GSTR-3B, you should reduce the amount of current month’s sale by the amount of such credit note.