Been made pay bill that is not mine

I picked a vehicle around early January 2021 and returned mid Jan. Before picking up the vehicle I noticed a few existing damages and noted them on the checklist sheet. Today Thrifty has debited my account with R16k for a damage that existed before I picked up the vehicle. I have been trying to resolve this with Thrifty for the past 3 month without any luck. I have asked Thrifty to have a look at their old rental contracts to establish where the damage occurred, however my attempts have fallen on deaf ears. Today I have been made to pick up a debt that does not belong to me. Today I queried Thrifty about it, and the response was that I need to provide proof that they (Thrifty) have debited my account. Are you kidding me! I'm suppose to provide proof of your (Thrifty) own actions! Seriously! What is really happening there? Are you guys running a business or what?