How can I take a blame for your system error???

System went off line while I was making a payment then it doubled the amount. I enquired on the secured chat about their system and and a possible solution but I was told the reversal will cost me R313 with a 14 days turn around time. Why must I pay for their system error, it's month end and I need that money for my debit orders. This is ridiculous to think that I have been asked to join Nedbank and standardbank private banking but I have been loyal to fnb for this ****, I'm counting my days and will ne closing this account.