Unfairly treated by Gezina Builders warehouse

I bought the garage door at Builders warehouse Gezina Pretoria, i wanted 3m door and they ssid 2.4 m will be fine. I bought it but when i got home i then realises that it is small. The challenge i am having is that they dont want to refund me because i lost my slip and bought it cash. I want hello Perer to assist me so that Builders warehouse Gezina can give me my money back. Even thogh i lost the slip, I can identify the teller who assisted me and the ssles person who assisted me. I feel like i am being unfairly treated and i even left the door at their premises.