Charge for imaginary damage to rental car

I rented a car from Avis at O R Tambo airport on 31/01/2020 and drove to family at Krugersdorp where I parked it safely on their premises. On 02/02/2020 I drove back to the airport. On arrival there the guy said that there is a small dent in the one door. Neither me or my wife could see any dent. Now I get charged R785-00 for "Accident damage". When I called their call centre, the guy just told me that it was shown to me and that they have an invoice for that amount from the panelbeater. There was definitely no dent.