A hard decision to make I can see. I can relate as I'm in a similar position as an entrepreneur. If I had an offer like that, I'd take it. As much as I discourage lying, call in sick. Yes it's a risk of getting fired and loosing out in the long run. However, you want to get your name out there, and that's when business starts. When a client is willing to fork out big bucks, there's pretty much a big gossiping mouth that comes with it; and that works in your favor. They know people, and people who know people. Plus the congregation you're performing to. It's big exposure. Personally, it's worth the risk. Starting out, there will always be risks, which right now seem big because of your financial position; but the rewards are far greater.

That was kind of a dumb move, advancing your music career would've been the way to go. This $2000 gig could've led to more gigs at the same price point!

Hopefully it's not too late to change your mind. Most part time jobs, which I'm assuming this is since most gigs are at night, are actually pretty chill about important career advancements if you have a good manager. My manager has always let me do something that's important school or career wise because she understands that the part time job isn't meant to be a forever thing. Maybe just talk with you boss and explain the situation, you might be surprised at how nice people in authority can be. Unless your boss has already proven themselves to be an ass, then just call in sick and say **** it.

Take the gig. Chances are it will lead to similar gigs which will lead to you not needing that job or any typical job. In the end isn't that what you want? To do what you love for a living. Advice from a fellow musician.