Dispicable service

So today I went to fill up at the Engen garage in Malelane Mpumalanga. I pull up and tell the attendant I want "95 for 100" meaning 95 unleaded fuel for R100, every where I have filled up before (which includes the garage in question) I have always used the same terms and I've been understood. Today the attendent heard R500 instead of the R100 I requested and went on to fill up for R500. At the time he's filling up I become busy with something else so I don't notice this, so now when I'm paying he asks me if I hadn't requested for R500 fuel, so I set him straight on what I said... Long story short I request the manager in hope of finding a solution of what must be done because now I was standing there going back and forth with the other attendents who obviously took the side of their colleague, I even suggested for them to drain it out which they tried but weren't successful for some reason. So the manager arrives he doesn't even greet he straight out made an accusation saying "it seems you're causing problems here", so now I'm surprised that he's gonna come at me without even hearing my side of the story and he's gonna be sure that I'm the one in the wrong. He took an accusatory tone where he said "there's a huge difference between 500 and 100, so basically I must have done it on purpose. First of all this guy doesn't take my version of events and secondly he's pretty sure that i did something wrong. I've never been so infuriated in my life, for a manager ********ise a customer like that is unacceptable. The situation was not only inconvenient for them but for me as well because I spent something close to 45mins going back and forth with these guys and eventually I had to pay for R400 of fuel I didn't ask for because someone misheard me. Having to pay wasn't even the real sting, it was the ******** manager who condemned me of theft without getting the whole story. That is unacceptable because I've never ****** a thing in my life so I don't know what basis this guy had on assuming what he did because he doesn't know me, or is it company policy to assume people guilty until proven innocent? He handled the situation poorly to say the least.