Changing cotract without my consent

I rented a car on unlimited milage. I received a Nissan Micra A few days later I needed to return the car as the petrol cap was broken. While renting the car I was needing a bakkie to deliver a water tank. The Tempest team where very helpful in helping me secure a bakkie for 3 days. I asked if I could leave the car with them while I used the bakkie as I couldn't afford to return it only to book it out again when I returned the bakkie and have to pay a third deposit. They said they would make a note and see if they could not bill me for the car the days it stood by them. All went well and i returned the bakkie and took the car again. After driving on a dirt road I noticed the front wheel making a knocking sound when braking. I phoned they said I must exchange again. Went to PE to do the exchange only to be told they had an unexpected large amount of booking and couldn't assist me. Had to get back to Coffee bay where I'm working so they said I could exchange in EL. They exchanged the car and gave me a Ford Figo. Soon realized I was spending an extra. R100-R200 on fuel a day so returned the car. When I received my deposit back there was R2600-00 missing. They had changed the contract