Unknown charges

On my Gold Cheq Acc it is sitting on a minus of R880 and no1 can explain to me why, when there was money there swiping was done, it went off because of cash being in the account, now I am told it was approved but didn't leave the account. Yet the money was debited and placed Into a pending debit account, now I'm charged for unpaid debits yet when the transaction took place it was catered for. I need someone who understands how the fnb system works to contact me, why should I be charged if fnb did not process the transaction yet it says approved at the vendor.