Wont let me take another item because i misplaced

On the 14th November 2015 I bought a shirt at Foschini Richards bay cash. I bought a size 18 which was the last biggest size they had. When I wanted to return it I could not find my slip. When I went back to Durban, I called Richards Bay Foschini and told them about the item not fitting me and that I had misplaced the slip. The lady I spoke to said I can go to any Foschini I can exchange the item for any other items but I cannot get a refund because I have no slip. When I went to Foschini in Westwood on the 12th Dec 2015, I was told that if there is no size on the item I had originally bought then I cant take any other item. The item still had a tag for crying out loud.