Terrible service and damage to my car

I took my car for first 19000km service at Centurion Mercedes Benz. Not only was service consultant inefficient but she was also rude numerous times . When I received my car back there was a chip on the drivers door. I complained about it and they finally agreed to fix it. I was informed they would provide me with a loan car while they fixed the damage they had caused. In the contract I recieved from them it stated I was responsible to pay R40000 excess should anything happen while I drove their car. I was very upset that I had to take such a risk when they caused the situation but I had no choice as I needed a car. The car they provided me was filthy inside and out and without any petrol- only just enough to reach the nearest garage. It was difficult to see if there was damage to the car but the service consultant assured me there was no damage. I was exceedingly careful while driving this car and when I returned it the servoce consultant went around the car while I stood there and told everything looked fine on the car and also told me I didn't have to sign anything. I later got a call from the same service consultant telling me the car rim was damaged. I have never received such degrading and poor service. After everything Centurion Mercedes Benz has caused they now have the audacity to accuse me of damaging their loan car. I am ABSOLUTELY certain I did not damage their car while I drove it. What happened after I left the dealership and the car (which I was told was fine) I do not know but am shocked to be told this is my fault. I cannot in good consciousness accept this and I feel people should be warned of the terrible, inefficient and degrading service Centurion Mercedes Benz provides!