As per GST tax is not levy on Discount as per Section 15(3) so the question is like Big Bazar they sold the Cards in Rs. 10000 Plus GST and allowed to purchase against this Cards Worth of Rs. 13000/-
In that Case they Deposit the Tax on advance Money on 10000/- and after that they sold goods of Rs. 13000/- Rs 3000/- is Discount ( Free ) GST whether the GST is liable on 3000/- or Not.