GST on Cash Coupans

If a supplier issues a credit note to a recipient of goods against a supply made to him for the cash coupan which the recipient has found out from the packet of supplied goods. If there will be any impact of GST in this transaction ?.
Let’s say cost of goods supplied Rs. 1000 + 18% GST = Rs .1180
Recipient found in the packet a coupan of Rs. 100.
Then the supplier issues a credit note of Rs. 100 to the recipient and the recipient pays to the supplier Rs. 1080/-
My question is if this Credit note is liable to GST or not.