A few months ago I quit a karaoke gig after waiting a month and a half to get paid and they tried to pay me less than what we'd agreed to.
The manager wanted me back and asked me what it would take. I told him I wanted almost double the pay and I want it up front. When I get there, if there isn't a check waiting for me I'm out.
About a month ago I got a message from him saying he got it in writing from the owner. I came back and after a month they've held up their end of the deal.
Don't be afraid to put your foot down.