This is cash discount (for prompt payment), so GST will not be charged in credit or debit note.
GST is applicable when there is a supply and in this case there is no supply, so GST is not chargeable on this case.
GST will be charged in Debit Note or Credit note only for trade discount.
So answer to your question is:
- GST is not chargeable,
- No need to pay GST