Refusing my refund and charged twice for PEtrol on vehicle used once

This Company is a ****m ,I returned their Vehicle and refusing to return my deposit ,which rental law state that ? ,Overcharged me petrol twice on vehicle used once ,I paid upfront Petrol of R900 and on return vehicle was refilled with R544 ,now I am charged both R900 and R544 refill.

They are refusing my deposit because I returned the vehicle before initial time .Sent me email to come and collect the car for two weeks ,I then refused ,now they are refusing my R5000