Post sales discount treatment under GST: substance determines whether discount reduces supplier value or is taxable consideration for services. Post sales discounts fall under clause (b) of sub section (3) of section 15 and are treated based on substance: unconditional discounts relate to the original supply and may be excluded from the supplier's value if sub section (3) conditions are met; discounts contingent on dealer promotional activities constitute consideration for a separate service by the dealer and attract GST with corresponding ITC to the supplier; payments enabling reduced customer prices form part of the dealer's value of supply and affect the customer's ITC entitlement; where discounts cannot be excluded and financial/commercial credit notes are issued, the supplier's original tax liability remains and the dealer need not reverse ITC if he pays the reduced value after adjusting the credit note and original tax charged.