Mr Price total ripoffs

I bought a pants which was a size too small. I exchange it and at the till the cashier tells me that the item won't go through cos there's no credit on the account. I had to make payment first. What happened to my credit that I still had on the account? Never have I ever heard that can't do an exchange cos there's no credit on the account. After I threw a hissy fit, only then did they put it through without me having to do a payment and that I won't get a receipt for the exchange. They charge a lot of interest on their accounts, that's why when you want to buy again you find there's no more credit cos they took it all with their interest.