avis not playing fair

In September 2013 I rented a car from AVIS, cape town international. I stayed in a the cape town lodge hotel where I parked the rental, one morning whilst reversing I clipped the left tail light on the wall which resulted in a cracked tail light and a scuff on the left back bumper. They sent me a bill for over R7000 claiming I have been driven in from behind and that the bumper and inside spare wheel was so severely damaged it needed a big job - which of course I disputed. After many back and forth emails we came to an arrangement that I would settle with them which I did as per the outlines of the communique. Later I was handed over to credit works - I forwarded all the mails and information to them. Today I got a call saying I still owe R4147? How is this possible?