If a guy is a high demand engine builder and NOT a 'businessman', he will eventually be so backed up on engine orders and behind on bills, he starts using parts monies from one customer to pay parts bills of a different customers engine.
My brother let this creep up on him a few years back. A genuine high po engine scientist/builder, but NOT a businessman. A bankruptcy, mad customers/friends/associates, a near divorce, and mental stressed to the max. He's paid back 98% of what he owed his customers outside of the bankruptcy, but it's taken 10+ years.
He's still building lots of high HP engines for folks, but has taken a completely different approach.
Sorry to get sidetracked, just relating a true story why you should get EVERYTHING in writing and document the progress of where YOUR parts/money is going.