Avis blaming us for damage that was already on the car

I have rented cars from AVIS for year, the week of the holiday we hire a car for 10 days. I took insurance for the first time worrying about the floods and road. We arrived at night when shown to the car it was in a dark corner, Anyhow we drove off with the car, When going over bumps the car felt low touching the under peace so we were very careful with numps of the car going over bumps we thought nothing of it, On returning the car the attendance receiving the car look straigh under the car and said look at this. It had the under piece of the car hanging on one side. I told the guy that was not us. We have just received an sms for an amount of R5217 being deducted from the credit car. No call no contact was it us, when I adamently said the car was like that. How do I get this investigated am really Not happy. SO in other words dont take insurance they think you are a target