FNB not taking responsibility for system error

FNB's system is throwing an error of insufficient funds when processing an online payment. As I have more than sufficient funds I processed the payment twice. FNB now want me to complete documentation accepting liability to pay R313.00 and a time period of 30days to revers this duplicate payment. After signing for this liability and agreeing to these terms, they will then request a waiver of the charges. Seriously FNB. YOUR FAULT, YOUR ERROR, YOUR PROBLEM TO RESOLEV THIS with no liability to me and within 24 hours.