poor service

I had recieved a R 100 voucher to use if I spend more than R 300, last month dated 27 May I purchased a Guess t shirt size medium, I then went to exchange the t shirt for the size large and was told by the teller and manager that I cant reuse the voucher, hello this is an exchange for size, is this how poorly the store treats customers, i was so upset I had to just cancell the purchase.