Did not get exchange

I went to South Gate Game to purchase a waiste belt. On the box it was 2XL, so when I got to the till I asked them to get me a size Large, the lady left to go check for it, came back and scanned the 2XL box, then I asked her again to please not forget that I want size large. She then left and came back with the box and when I got home and opened the box, the belt was too big. I didn't even use it because it is too big so my boyfriend went to go exchange it and told them that they gave us the wrong size even after we repeatedly asked for a size Large. But they told us they can't do anything. This item was purchased on the 11th of November. I did not use it, I only opened it and saw that it was beyond my size so why would I wear it. What I would like is an exchange because that belt is R300 and it doesn't fit me.