on discount

GST not applicable provided the corresponding credit is reversed by you [Sec. 15(3)(b) of the CGST Act, 2017].

You have metion the Credit note details in the GST return in the month on which you are raising the credit note. ( subjet to the time limit specified in GST law).

For further details you may refer section 34 of CGST Act,2017. details out the the provisons of DEBIT/ CREDIT NOTES