Help

I bought a ****** Mary dress ( ***3) for my daughter at Truwoths Soshanguve Crossing on 03/08 which costs R850 and redeemed my birthday voucher of R100. Transaction # ***. Today, 04/08 I went to exchange the same dress for a smaller size and was told to add R100 as voucher only works once. I don't understand why because it's the same dress, same code and same price. I requested money back and couldn't get it because I didn't have the credit card used for the purchase. It's just stories and I don't have the time. I hereby request for my voucher to be reissued or refreshed for the exchange or can I have my money back please