True, someone who is in a bind over $20 tickets is not in a position to put down a fast food job. But when it comes to heart stuff, logic doesn't always make it all the way around.
I suspect that if she put herself out seriously to buy tickets to a special show, her feelings would be hurt on some level, no matter for what job he turned it down. It's kind of like when a little kid offers candy to a grown-up, and the grown-up says 'no thank you.' Even if that grownup can explain to the child that he doesn't eat candy because it makes him sick, the kid will still feel sad, and may need extra hugs, or the chance to share something else.
It's still a rejection on some level, and rejection always hurts, even if you know why. But Nos, you're a good guy who loves his girl. So I'm sure you'll find a way to make it up to her, and she will understand. In the end, if you went and lost your job (that you need right now) over this, she will feel way worse about that, than if you don't go to the show. Do what you have to do, it'll be fine.
Odrade