Maybe I won't go looking for a job tonight.
Really, the boss can demand money, at which point I inform him that is illegal. If he persists, I contact the state department of labor, and file a complaint. All hell breaks loose.
He could try to cut my pay, I think that might be legal, but then I'll give him my two week notice. He could attempt to forcibly rearrange my shifts, which is legal, but then again, two week notice. He could reduce my shifts, in which case, again, two week notice, and also refer to "fire". Which is to say, he can legally fire me, at which point he doesn't have enough people to run the store at night without himself coming in multiple nights out of the week.
So therefore, he's stuck at a point where he can't really fire me. Even if he hires multiple new people, it'll take time to get them trained up, going into the busiest season, so he's unlikely to try anything until after the season, by which point I suspect he'll have forgotten about it.