FNB EFT reversal

So I deposited R500 to incorrect account on the 29th of May 2021. I contacted FNB only to be told that I will be chard R313. How is this possible why so much money. I am really disappointed at FNB for ripping poor people off. This money was for maintenance to my step son. I just ended up telling the agent to never mind. He doesn't have to reverse it any more. I mean what's the point of reversing and at the end I will get peanuts back. FNB is the worst bank in the country. I want to move to something else. I am hurt broken.