False Accusation of damages

I hired a hyundai I20 from Avis from the 22 to 24 may 2015 and was falsely accused of damaging the car. I received the car at 10pm and the lady Natalie told me that it was pre inspected and shouldnt worry...so i checked around the car by myself and obviously with my untrained eye and being at night didnt find any problems. i hardly drove the car and it was parked off safely for most of the time that i spent in durban so when i returned it i was confident that there was no issue. ...on returning to jhb i receive a message that the car was damaged. i called them and now they accusing me of causing a dent underneath one of the doors. This is really unbelievable and so untrue coz i would've definitely known if i had hit something and wouldve reported it...and with the car parked off for most of the time theres no way i couldve dented it....the dent was there before and unfortunately i didn't pick it up when i received the car and so now i have been accused of damages. ..