FNB refuse to give refund of money erroneously paid into my account.

A person paid into my account in error. He requested a refund from FNB as soon as he discovered he did so. FNB refused to give him a refund stating "insufficient funds". Now this may be so as I do have an overdraft but FNB has no right to refuse this person his money. I have not utilised it at all and he has every right to get a refund. Now he is asking me to pay him back and I cannot do so as I am in a very bad financial position at the moment. FNB has no right to make him suffer due to his error and make me responsible to pay him back. I would accept responsibility if i USED his money but I did not. The only solution I have now is to go to the Ombudsman but I am truly hoping FNB will solve this matter before I do so as this will also be time-consuming.