Disappointed Customer

I bought an item by Foschini in Kimberley Diamond pavilion and used my discount rewards voucher which is actually something nice when you make a purchase. But the next day when i went to exchange it for a smaller size i was told i must pay full price because the voucher has already been used. I fail to understand why the price must now change when you are just returning for the very same item of a different size. Then i get told over and over again that the voucher has already been redeemed like I'm some kind of stupid. I totally understand what is being said but i still believe that they should be able to override such a transaction or put something in place whereby the price is still the same coz afterall that's what you paid for the item. I am a faithful customer of Foschini for like 30 years and I have NEVER missed even one installment hope someone checks that up, but really not impressed with this voucher problem that cannot be overridden.