unfair treatment and being ****** of my money

I had bought some clothes during monthend of October 2020, I unfortunately lost the receipt but I paid using my bank account. 2/3 weeks in november I went back to get a refund (worth of R119.99) because one of the items didnt fit me(it was too tight). I was told that I was going to be refunded R80 because the item was on promotion and when I bought it, it wasnt on promotion, I dont have a receipt of which I refused because I paid more than that, I had opened my banking app to show proof of what I had paid but it was disregarded. I asked them to check the transactions of that date they will see how much i paid, I remember it was after work and at what till I paid, they refused to help me. This is day light robery for Mr price to expect me to accept less money than I paid just because I lost the receipt? They disregard all other proof (bank transactions) just because I didint have the receipt, I told them their cashier didnt ask me for email address to have the receipt emailed to me either, why am I being punished for shopping at Mr price, on top of that their staff was giving me attitude that day, I was at the que for over 30 minutes because I didnt have the receipt and asking them questions as I didnt understand their policy I was treated like a nuisance, cashier kept asking me to step aside because she wanted to help other people at the que as if my issue isnt important and I am just annoying them and they got better things to do, I no longer want to shop at Mr price after this if this is how you treat your clients.