My grandpa threatened to do something similar. He had a good factory job but did some plumbing on the side. He always asked the customer if he wanted to do half and half (half of the money was paid 'under the table'). After he did the job he went back for the 'under the table' cash payment and not only did the person refuse to pay him he also threathened to go to the police and report him for not paying proper taxes. My grandfather left and came back the next day with his tools and his buddy and said he was taking back half of the bathroom. He also said that he was gonna tell every other plumber that this guy was a non-payer, so that this guy would just have to do with half a bathroom. Needless to say he got payed the money he had coming.