Diplomacy and have all your ducks in a row.....This is really an opened ended question and requires specifics. But the best thing to do is go over your records and know them throughly just in case diplomacy breaks down. And when I state diplomacy, I mean having a sit down conversation and really communicate what the problems are without name calling, accusations, editorializing. In fact, a mediator is also a good idea. If this was two large corporations and the business arrangement was breaking down, then I would fully suggest lawyers on both sides. If this was a person to person arrangement, avoid lawyering up. The cost is too much and stiffens negotiations. Have a mediator there and above all else, try to remain calm. Lawsuits and lawyers should always be a last resort, never a first. You could end up waiting 2 or more years for a problem to resolve that could have been accomplished by a simpler sit down meeting.