wrong change

I was given the wrong change at a service station yesterday, unfortunately i didn't realise this until i checked my purse later in the evening. I called the service station as soon as i realised, the man on the night shift was dismissive. I asked when the manager came in - 6:30 am but he said not to call before 9 or so because then the tills will be counted. I'm going to call at 6:45 and ask her to call me back when she's done the adding up. I'm pretty peeved because it was a $100 note that i gave her and i only received change for a $50 note. Does anyone have any suggestions as to the best way to approach this? Any help is appreciated.