Lost slip and not able to get a refund/return items

Really upset with Mr Price, I bought items in store (at Irene Mall) in December and lost the slip. The items don't fit and I went to the Festival Mall Branch to request a refund, which I was told is not possible but I could exchange the items. So I left the store due to time constraints and returned after a few days to exchange the items but the manager refused saying, I cannot be assisted at all without a slip. The tags are still on the items surely this should help identify when the items were sold etc. this is frustrating and basically means I wasted my money.