Bad Service

I bought my husband cycling pants as a present, he did not like the pants so he said that i could return it for something different. I lost the slip. I went into Mr Price and was told that the pants were R199 and that I could choose something to that value, if it is less then they will give me a voucher for the balance. I chose a shirt for R130 and went to do the exchange. After struggling with the voucher I was informed that the pants is now on sale and is R99. I was informed that I need to pay in for the shirt. I find this unacceptable! Yes I do not have the slip anymore but that does not change the fact that I paid R199 for those pants. I am disgusted!!