ABSA, the bank who takes money which does not belong to them!

ABSA Vehicle Finance took an extra instalment from my account after my previous vehicle was fully settled by the dealership. I sent proof that the money had been taken out of my account by ABSA but they now deny it. I sent the settlement letter from FNB to ABSA as well showing that the vehicle was settled. I have dealt with about 9 different agents and not 1 can give me an answer except that their was a system error in the beginning of November. ABSA has taken R5700 from me which does not belong to them!