Poor service at Truworths Galleria Mall

I bought a wrist watch last week and i used my Truroyalty voucher , when i got home the watch wasn't working , so i immediately phoned them and i was asked to go to the store for further assistance, i went to Truworths Galleria and the Ladies there just decided to give me my money back even after i explained that i wanted an exchange and not a refund , their answer was they cannot help me since i used a Discount voucher and it has already been redeemed but now my thing is .. whose fault is it that the watch was faulty? So i must pay extra (for the same watch) because their watch is faulty???