Presuming this is a fancy sci-fi kind of anarchy: multi-sig escrow wallet requiring a 2-of-3 quorum of oracle triggers from observation cubesats owned by both parties plus some disinterested third party.

(People are already doing the virtual equivalent for prediction markets: everyone who cares runs their own oracle agent to forward result data from web2 sources into web3. You “just” need computer vision + good optics to bring that paradigm into the physical world.)