The only suggestion I can make is to check if there is any kind of security camera/CCTV aimed on this machine. If so, you may have a slim chance of them seeing that you didn't receive any money.
I know it's easy to be wise after the event, but I would have grabbed the nearest passerby and asked them to confirm that my purse was empty.
If they're saying there was no surplus, I don't think you have any chance of them giving you �50.
You could always complain verbally in a very loud voice in front of other customers.....or threaten to go to the press. That may make them back down, to avoid negative publicity.
Good luck.