Still waiting for my deposit to be refunded

When I picked up my rental car, it was totally different to the car I booked although \apparently"it was in a similar class. But what bugs me is that the agent informed me that I have to pay a security deposit. (Which I knew I had to do) My complaint is that she said it would be refunded on the day I return the rental car. It has been more than a week and I have still not been refunded. The accounts department has also givin me a invoice for paperwork. Why should I pay for paperwork that they process? There was no damage to the vehicle. The tank was filled. When I took the mater up with them they responded by saying the matter is under investigation. What is there to investigate? """