Given a voucher only to be told that I forfeited it

I was given a R100 voucher and I used it today at Sportscene maponya mall. I bought a T-shirt and I couldn't fit it at the store as it was packed and I was in a hurry. The T-shirt was small and I realized that when I got home. I went back to change the size and the lady who was assisting me (Gloria as written on the receipts) told me that I forfeited the voucher discount as I already used it on the t-shirt I am exchanging. I struggle to understand because I was exchanging the item not buying another one. I am so disappointed with the service because was I told that before I bought those clothes I was not going to bug them at all. I am done with TFG I just want to finish paying off the account and cancel it.