My approach would be to get this guy to understand that he needs working capital, not permanent capital. If he needs to stretch 50k over 2 years, what's going to happen when he needs materials or labor for the next job 4 months down the road?

I would sell him a smaller amount over a shorter turn and utilize it like a line of credit. This way he can comfortably afford the payback and can get capital on an ongoing basis. The cost of funds is cheaper for him and if he needs to break glass in case of an emergency, he should have the revenue to buy this out and do so.