Exchange policy and recycling clothes

Purchased a pair of jeans and a top at Eastgate Mall and made use of the new recycling programme which meant I got 15% off for each item. Fitting rooms are closed therefore I was unable to try these items on. The top did not fit and I attempted to exchange for a different item at East Rand Mall and was told I am not able to use the discount voucher previously received. I felt this was not fair because I was not going to receive the bag of clothes I donated initially.