Charged for apparent damage

When we collect the vehicle from Durban airport we were not requested to check any damage on the vehicle before we took delivery. When I delivered the vehicle back they said there was an apparent mark on the drivers door. I signed an affidavit to confirm we did not damage the vehicle and also that we were not given the opportunity to check for any damage before we took delivery of the car. Out of the blue i receive an invoice for the damage to the value of R 2, 239.92. If Europcar does not bother to have us check and sign off any damage to the car, how can they just assume the damage was caused by us. It could have been the previous renter (as they obviously don't always check the cars) or it could have been damaged in the parking before we took delivery. I put the responsibility on Europcar to prove to us that this damage was not on the vehicle before we took delivery. I will definitely not be paying for Europcar's lack of attention to the renting process.