FNB ****s clients with Credit Card "system error"

My fiance an I have been loyal FNB clients for many years. In 2021 he upgraded his account to a Private Client account via the FNB app. After the upgrade, more funds showed up as available on his credit card to use. We assumed it was because he upgraded which resulted in his credit limit being increased. These funds were very welcome during that time as we were going through a difficult patch with sudden pay cuts at his work. After a few weeks the remaining available funds just disappeared so we went to an FNB branch to inquire. After about four hours there they told us they could not help and that we had to phone the help desk as well as send an email. We got no feedback nor a reply on the email. After a month or so he started receiving calls from FNB instructing him to pay back the whole amount of money he used as it appears to have been an error on their system and was not a credit limit increase and is now seen as an overdraft. He told the people that phoned that he will pay it back but that he cannot do it in one installment as they request as it is more than his entire salary. Everytime they say they will get back to him but nothing. Each time a new person phones and it is the entire conversation on repeat as they are not clued up on what was discussed on previous occasions. Now FNB has handed him over to collections and he told them the same story; that he is NOT refusing to pay, only requesting that they come to an affordable agreement in which he can pay it back seeing as it was a system error on FNB's side and not his fault. Once again he is not receiving any feedback. We have become aware that the same has happened to other persons besides us. We will both be switching to a new bank as soon as possible.