Audi centre westrand sabotaged my wheel

I took my car in for a service on Monday, 9 December. It had been making a funny noise. They returned the car, having ordered the parts to fix my leaking right side engine mount. On Wednesday, while driving in a suburb my car broke down. The tow truck driver pointed out that the front left wheel had only one nut fastening it in place. Someone picked up the other nut on the roadway. We towed the vehicle to Audi Centre Westrand. I was in a state of shock. And yet, the service centre still managed to squeeze me for five hundred bucks for new wheel nuts! Thus, not only did they NOT pick up the fault when they inspected the car for worn front brakes but they also had the gall to make me pay for their oversight!