SANI SIXT Randburg charged me a rental for 12 days when vehicle was returned in 4 days

I rented a vehicle for 12 days because my vehicle went for some repairs, after 4 days i got a call that my vehicle was ready for collection of which we went and picked it up, we then returned the rental vehicle on the 4th April, on the seventh i call SANI randburg asking since i returned the vehicle early would the 8 days be subtracted from my invoice and i was told no i still pay the same amount that i paid for 12 days, then i asked if i can get the vehicle back then because the main reason i returned early was so that i pay a little less than what i would have paid for 12 days and they refused to give me the vehicle back. so i have paid rental for 12 days for a car that was returned in 4 days which is a total *******. i will never use them ever again.