FNB Withholding Money That Is Not Theirs

I rented a car in America (Avis) and used my FNB card to release the vehicle since the company card had not been opened for international travel yet. Avis assured me that the transaction could be moved to a different card upon the return of the vehicle. Three weeks later the car was returned and the transaction moved. The car has been paid in full yet FNB won't return my money. Avis has assured me that the money has been released but the bank won't make the funds available. FNB has been beyond unhelpful and are now in the realm of obstruction as they want me to go down the dispute declaration route. It is unbelievable that I have a substantial balance in the bank but no access to it. FNB refuse to return my money. It's shocking. And it's been this way since 16th March.