Asklemmy

Nothing you can do but go no contact (0%). He needs a time out and to reevaluate. You must be ok with whatever outcome that lays ahead since you cant control others, you can only set a boundary that says you won't be discussing this or tolerating anybody who continues to revisit it.

They won't like having control (control of You) wrestled away, particularly if they're accustomed to that, so it might take them a while to adjust their behavior but it is necessary to resolve that dynamic. Make it clear that there will be no money talk or transactions flowing between you and if it comes up again, you will be taking a break for a month and then everyone can circle back and try again at that time.

If it happens again after you've had to enforce a month timeout, you have to decide if you want to do escalating consequences or if you want to start a full and longer-term no contact regime where they are completely blocked and unable to communicate with you any longer

If its escalating consequences, start at a lower threshold like say a week, any violations that happen after they've been welcomed back, adds to that or doubles it. I suggest doubling so

1. 1 week
2. 2 weeks
3. 4 weeks/month

If they refuse to respect it after that I would consider strongly making it permanent. They are adults and that is literally an extended tantrum they are and continue to throw to your detriment and without seemingly intelectually recognizing that they are not going to win this or beat you into submission somehow. Do not depend on them for anything or give them any bargaining chips. Once you impose a consequence, do not falter or they will sense weakness and persist or get worse.