Incorrect billing, unprofessional behaviour and dishonesty

I rented a car from avis Polokwane international airport, unfortunately the return date was on a holiday which was the 16 December 2019,I contacted their head office to enquire what was going to happen and they advised me to return the car and leave the keys at their key deposit box on the branch and I was concerned and asked how will they know how the condition of the car was when I left as they won't be any one to check and they told me there are cameras and should there be any query the footages will be provided,they then deducted my money claiming that the car was damaged and when I went to their branch they said they were doing investigations n will get back to me ,it's been over a month and I am not getting feedback and when I contact them they take me back and forth and their consultant s hang up the phone on me