No refund for merchandise now on sale

On 08/12/17 I bought flops at Mr price southgate forR136.99 . Today I had to return them because they are damaged in less than a month. I do not have a slip because I misplaced it however my card transaction history shows the transaction. The shoes were changed without any hussles and when I told the lady who was helping me that she needed to refund me the difference as the shoes are now R70.00 she said she could not do that as I did not have a slip if I had a slip she would be able to help. Again she did not give me a slip today saying she exchanged the shoes and I dont need a slip for that as I didnt give her one . This is utter nonsense a customer pays for the price on display nothing more nothing less..