Very unhappy on exchange of my Christmas gift

I went to Mr.Price pavillion to exchange a christmas gift I received of R199.90 as the tag was still attached for this reason. I wanted the exact item in a smaller size as I loved the gift but it was too big or me. At the till the original gift scanned only R119.90. When I asked the cashier why he said it is because I do not have a slip so they deduct a total of R80 for that reason. This is rediculous. I went to read about their return terms and it doesnt have anything stating that the price comes down on a return with no receipt to show.