Harrassment

In August 2014 i hired a vihicle from Avis and apparently it had a scratch when i returned it. I was asked to pay R2650 for the damages and i was not willing to pay at first because i knew the scratch was already there when i took the vehicle. I was told i was going to be handed to the legal department and i was told i was going to be responsible for the payment. Then i panicked and immediately paid them the money. Now 3 years later (2017 September) i receive a call from the company called MBD saying i am owing them R600 because i only settled money i was owing to Avis and the money has grown to be R850. I told them i do not know their debt if i knew anything about it i would have settled it. Now they keep on calling me at work and they are harassing me with messages almost daily. The lady i spoke to yesterday told me that the money will keep on growing and i will never ever get another job or credit in future. A friend of mine advised me to contact council for debt collectors, i really do not know what to do. Please assist.