No return without slip policy

My sister and her boyfriend bought me 3 dresses at Mr Price in Worcester CBD on the 25th of November , but I didn't like one of the dresses. I tried taking the dress back on December 1st for exchange but couldn't find the till slip. The lady at the door said they are unable to do exchanges without a till sleep even after I showed her the proof of purchase since they paid for it with a card. I am utterly disappointed since I will not be able to wear the dress. Please consumers keep your till slip