Damage on car bumper.

Rented a car at Avis Cape Town Airport from 17-23 September. After a week they let me know there is damage on the bumper and I have to pay R1352.02. On the day of returning the car and walking around the car with 3 other people there were now damage. They send me a photo of a scratch on a back bumper. This is an old scratch marked and I have now proof it is from the car I rented. How can they charge you for damage after you have done inspection on the car with them on returned? Now that I am not near the car they decide i have damage the car. I now there were no damage on return. This is again the bumper ****.