Refusal to pay deposit after 3month-contract ends

Hired a car from Dynamic Car Rental aka DSCMC in midrand,the lease clearly states its 3moths and even states return date(4th August),upon return(30th September) which is the end 3rd month end,they refuse to pay back my deposit as agreed their reason being I needed to give a full month notice????HOW cause the lease has lapsed and I don't want to renue it as advised that I should let them know only if I wish to continue....

I took care of the car they where even shocked of the low millage as I never traveled with it as I needed it only for emergencies, I NEED MY DEPOSIT BACK!!! I HAVE THE INTIALLY SIGNED CONTRACT WITH ME.IN HARD COPY