GST DEBIT /CREDIT NOTE REPORTING QUERY

First query Best way amend the invoice in GSTR-1 and issue fresh correct invoice with the same invoice number. Alternatively you can also issue a credit note

Second query again you can go for correction or you can issue a debit note. In this case you will also have to pay interest

Under rate difference to levies Debit Note shall b apply.

That means we need to issue credit note for 6% Rs.9000 in first case and debit for 6% Rs. 9000 in second case? sir there is no option in gstr 1 only credi / debit note for gst portion difference what to do?
sir why second situation need to pay interest? what is the rate of interest? interest is applicable on differential amount?(Rs. 9000) full amount(Rs.27000)

Go for amendment

And yes interest @ 18% on differential amount

That means in situation 1 and sutuation 2 credit/debit note is possible, we need to amend invoice by mentioning same invoice number because gst credit /debit note only for gst portion is not possible in gstr 1? am i right sir?
interest 18% is sure in situatuon 2 is sure?
what is rate of interest on credit note itc reversed in the same month if it is reverseed in the subsequent month?