Allowing debit that was credit back.now sitting with -balance in my account.

Left my account with -R3700.all i knwo is that if company tried to debit and they find less amount debit will bounce. But FNB decided to allow debit while the money was less.I got wrong billing invoice from MTN on the 17th then I was busy with dispute with mtn hence I transfer money to other account. And MTN advise me to pay in cash the one I know since I have been encountering of wrong billing since Dec.i paid MTN aR550 using my card .I was waiting for outcome of which problem because the date for debit was already on the system advise was to make sure the is no money or less money debit will bounce. I don't know why FNB allowed debit at my expense. That money was credit back by MTN because it was their mistake. It's credit back pls money from FNB. I tried to make dispute and reversal using app but because of maintenance I didn't. May 2019 my way tried to debit before actual date by they was no money debit bounced back why did they allow debit of such amount to went through worse at my expense now it's minus worse I left R730 in my bank even that one was given to MTN. Is not their problem if debit bounced back because there are many reason behind. Like this one matter is sorted between me and MTN they even charged me R11 because their debit was going to bounce while I was waiting outcome from executive team It was included on R550.I paid cash of R550 bank also allow R4480 to went through I m angry. All I was expected from FNB was to charge me feel of bouncing of debit not allowing the debit while the was no money. I m left with minus R3700 in my account because there was balance of R730 in my card. I ma now owing bank why did they allow that .worse MTN sorted the matter my crediting back that money I ws disputing. So angry about FNB .they will allow people do debit even if there is no money in the account.