What the OP has essentially said in her most recent update is that Younger Son has asked her to not be a flying monkey (i.e., don’t talk to Older Son on his behalf). And OP is respecting that request.
Using a flying monkey is a very passive-aggressive and often manipulative technique and it can cause lots of issues. (flying monkey is a Wizard of Oz reference, btw)
As for the Older Son’s pattern of taking advantage, it’s kind of hard to know what that really means (because different people might have different definitions of that phrase) without examples. But generally speaking, if somebody is repeatedly taking advantage of somebody else, it’s up to the person who feels taken advantage of to speak up for themselves and tell the other person to back off.
Younger Son probably needs to work on how to set proper boundaries with Older Son. Depending on the person and the situation, sometimes trying to be subtle and hinting isn’t very successful. Sometimes with some people, you need to be as blunt as hitting them over the head with a frying pan.
So if Older Son really DID agree to pay Younger Son for the painting, it’s up to Younger Son to remind his brother of the agreement. It only takes something like “Hey, you know I’m still waiting for you to pay me for that painting…since you agreed to buy it, after all. Just a reminder in case you forgot.”
If Older Son has a pattern of pushing the boundaries w/Younger Son in order to try to get away with stuff, then it’ll be up to Younger Son to not let him get away with it. Even if Older Son makes a fuss about it, has an adult temper tantrum over it, yells at him over it, or whatever it is he does in order to ‘take advantage.’
People can only take advantage of you with your permission.