Overcharge for fuel refill even though the car had a full tank

I am getting the worst experience. I returned their car on Fri last week, all was confirmed to be in order and it was confirmed that I owe less than R2300 which was confirmed as the amount that will be taken from my deposit and I will be paying the difference. The next thing they are deducting more than R900 from my account and when I questioned this I am informed that it’s for refuelling, and I’ve advised them that the car was confirmed as having a full tank and that I went to BP garage close to the airport to ensure I fill up. Now I’ve been waiting for their branch manager’s response and my emails are ignored. Why didn’t they send me the invoice 1st before deductions? I didn’t approve this amount and I need my money back.