I think you answered your question in your own post. Talk to an attorney, preferably one that knows or specializes in contracts. You have already learned the hard way that it is worth paying a lawyer whatever they might charge to have a legally binding contract. I used to do they 3 payments thing until I realized that since I wasn't doing major projects, it wasn't worth the hassle of splitting the jobs into 3 payments. Since then I've been doing 1\2 up front and 1\2 when completed. If I do larger (for me $10,000.00 or so), or longer time for installation type of project then I'd probably break the payments into thirds.