I'm not happy with Hertz they don't want to pay my deposit back

I hired a car at Hertz Cape Town International airport on the 20/12/2022 , then when I was in Eastern Cape on the 23/12/ 2022 at Caltex Garage they putted Diesel instead of Petrol in all the cars that they serviced that day. Fortunately for me I realised it same day when I noticed the car was losing power. The car could drive but the speed limit couldn't pass 100 km. I went back with it to the Garage when I arrived there, there were other cars having the same petrol. I spoke to the Manager of Caltex who admitted that my car also had the same problem as others. I told him that the car i was driving as a hired one. So I called Hertz in Cape town informing them about what happened. Hertz agent asked to speak to the Manager of the Garage which he did. On the conversation the Caltex Manager explained to the agent of what happened and even admitted that they will take full responsibility of the damage of the car.

They even sent him some documents to complete so that the Garage can take full responsibility of the damage. Hertz agent told me that they will sent me another car from East London since this happened in Alice(E.C). after 2 hours they did bring another car and the towed the one that had a problem. They even told the Caltex Manager that Caltex will be responsible for the towing amount which the Caltex Manager did agree in it. On the 18/01/2023 i had to return the car back to Hertz in Cape town which i did. I dropped the car and they said everything was fine. On my surprise on the 19/01/2023 i received an email with invoice saying that they won't pay back my deposit of R7000 because of the damage on the car for diesel that was putted on the car by Caltex. I called the Manager at Hertz in Cape town wanting to find out how can they don't release my Deposit whereas Caltex did admit of the wrong doing. Since Then I've been trying to get my deposit. I've been calling them everyday trying to find out about the hold-up. The Manager is failing to call me or even update me on the issue. I believe now that Hertz is trying to rob me of my deposit. Whereas they know that they already claimed from Caltex aswell.

I'm very disappointed with Hertz and now i don't even know what to do to solve this as the Manager doesn't even returning my call. What I don't understand here is Caltex is taking full responsibility of the damage but Hertz they don't want to give me my Deposit back.