I have a client I've been working with since January on a CFO hire. It's now May. They have decided they want to hire a candidate for a 3 month trial rather than committing to a W2. He may or may not be hired at the end of the trial. I charged them an upfront fee but the remainder for the original retainer contract is due upon hire (W2). Since they want to do this 3 month trial, how do I bill them?