I am not sure if I would stay out of it as this could cause a serious rift between them over something relatively trivial - the cost of the painting. You are already involved since the painter son complained to you. You wrote that the older brother had offered to pay in the past so not clear why he thought this painting would be free. OTOH, the younger son did not set a price or negotiate. I would be concerned about him using this in the future against his big brother and so willing to intervene to avoid that.
Could you say something to older son like “How much did you end up paying your brother for the painting”? Possibly adding “I know he had folks interested in buying it?”
Or straight up tell younger son he was wrong not to set a price and let brother know he wanted to be paid. And that using the lack of payment against brother in the future is a really bad idea, since he wasn’t clear payment was expected.