Being told on email that a credit would be done to the total opposite the very next day

This is my second attempt on this Platform and also having escalated my issue to GM level, and still I have had no joy. The higher the query went, the worse the service became. Since my previous review I had the area Manager state on email that a refund would be done for a scratch that I still maintain was not caused by myself. The very next day after stating in writing that the credit will be done, she then sends another email stating that the refund has now been declined. How can someone go back on their written word/commitment. I then escalated my issue to GM level to also be told that the credit is now declined and they are holding me liable. I have to state that this is by far the worst service I have ever received in my life. This appears to be common practice from AVIS looking at the amount of negative reviews regarding similar scenarios.