Vishal Mega Mart — Paid an extra charge for the exchange of the same defective product

I purchased 16 curtains on 09.04.2022 with a 20% discount offer. Later on, I found that three curtains were defective. Therefore, I exchanged the same curtains with the same design and price on 11.04.2022 at Vishal Mega Mart, Ludhiana, Ferozepur Road, Ludhiana. But the staff charged an extra price I.e without discount price. When I told him that I am only exchanging it, he told me that I have to pay the price without a discount as a 20% discount is available on the purchase of 10 curtains, therefore, I paid extra charges. It is pertinent to mention here that if I purchased 16 curtains with 20% discount and out of which 3 curtains were exchanged then why did the store charged an extra amount for the same? My contact no. [protected]