[Jack winces at the question. He really does need money, especially with the wedding coming up, but he doesn’t want to be a jerk and turn the boy down flat] Th-that’s a very sweet idea. I’d like to do a portrait for you, but I’m sorry, I can’t accept payment that way.
[He smiles, brightening up as he offers] How about if you p-pay on installment? Whenever you have some money, you can pay me, bit by bit over time, until the painting’s paid for. [It’s not an offer he’d make to most clients, but he does want to help this time]