Charged for scratch on car

We always use Europcar for all our car rentals. This time we rented at cape town international branch. When we delivered the car all was in order. Hours after we delivered the car someone phoned my husband to say there is a scratch on the car. He asked them to mail him the card that showed the scratches of the car and how they could be sure that we caused it. They send a mail, but without the copy of this card. The car had numerous scratches according to the card we received with the car. My question is: if everything was in order when we delivered the car, how can they be sure that 4 hours later we caused a scratch on the car? Their personnel drove with the car (to fill the tank, wash the car, etc?). How can they say it is us after the car was handed over in good order? Please assist what to do? They are charging over R2500. If they say when you deliver the car back to them there is a scratch one could understand, but surely not 4 hours later after other people drove with the car? I saw other similar complaints on hellopeter from the same branch?