Dear querist,
Section 15(3) of the CGST Act, 2017 allows deduction of discount from the value of supply. The exclusion mainly depends on the timing of the discount i.e., the time at which the discount is given by the supplier. In other words, if the discount given before or at the time of supply, the supplier would be entitled to claim the benefit of discount if the same is disclosed in the invoice. In case where the discount is given after the supply is effected, the supplier would be eligible for the deduction of the same, if the following conditions are satisfied:
(a) Discount given must be established in terms of agreement entered into at or before the time of such supply;
(b) It must be linked to the relevant invoices; and
(c) Input tax credit pertaining to same is reversed by recipient. In the light of the above, the discount you are mentioning is liable for GST, assuming that the conditions (a) to (c) not satisfied.