MTN fix this!

I upgraded a contract at the Vincent Park shop on the 31 March 2025 to a R545 home data deal, on the 15th of May they deducted R765, then I went to the shop and I was told it must've been a system error so I called mtn and they say the shop must have made a mistake. I am not happy with that no-one wants to be accountable, I just want my refund and to be put on the correct deal.