incorrect fuel loaded

On the 25th of October 2015 at 12h30 at caltex garage 250 koeberg road rugby in milnerton I went to fill up my tank with diesel. As I was busy checking my oil and water I heard the attandent saying he added the wrong fuel in my tank which was unleaded. We called the owner Mr Moodley told him what happened and he admitted to the owner I have stipulated diesel and by my fuel cap it stipulates diesel. Long story short the owner Mr Moodley said he will pay watever the cost will be. I called out my mechanic as I don't want anyone else work on my van as it just came back from a R40000 repair. The bill is R2600 and the owner now refuses to pay as it's to expensive as his mechanic can do it cheaper. He is now going to get quotes from other places as i have to get my van fixed as I'm starting my new job on the 26th of october 2015 as a sales rep which I need a vihecle to drive to clients. I must now pay the mechanic as the work will be completed today hence the urgency and obviously it's a Sunday and call out fees etc. Why must I now pay out of my own pocket due to their negligents