Progress payments can be negotiated at the start of the project. They can be based on actual costs of labor and materials or an agreed to percentage of completion. This keeps both parties engaged and the project doesn't go to the back burner. It also gives you the opportunity to evaluate the job and make changes if required. As Mick53 stated it works for both.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine