This Is Not The Ancestral Environment You Are Looking For
I have learned a lesson the hard way, and wish to pass along what I've learned in the hopes that others need not learn it the same way.
For any agreement that is long term and important (define as you will, but anything lasting more than a few months and likely to entail over $10K would qualify IMHO) - PUT THE AGREEMENT IN WRITING.
This sounds obvious to the point of absurdity on the surface. We all know this already! But allow me to point out a couple edge cases.
If the agreement starts small (maybe under a thousand dollars, maybe only a couple thousand, maybe just for a month or two), but it starts to grow slowly over time, you will eventually become very uncomfortable talking about it. Because there was implicit trust when the stakes were lower, and asking for a formal written agreement now implies lack of trust. It does not matter. PUT IT INTO WRITING, or cut it off.
The agreement may be with someone you trust implicitly. A sibling. A lover. The person who saved your life. Implying you don't trust them by asking for the agreement to be put into writing would be insulting, and throw the strength of your bonds into question. It does not matter. PUT IT INTO WRITING. If they actually love you and trust you, they will want to have it put in writing as well, for your safety as well as theirs.
Perhaps BOTH 1 & 2 are the case. This compounds the difficulty greatly. Guess what? Yeah - Writing.
You think I'm being silly.
In a long term situation, the person you are dealing with today, who loves you and saved your life, may not be the same person you are dealing with in several years. Future-Them may have developed a drug dependency. Or they may not care as much for you, values do drift. Maybe you are simply wrong about them right now. (Humans are terrible at judging three things: Volume, Acceleration, and Character).
But even if they should change, you think you're basically protected. Because the tribe knows of your arrangement. Both of you have spoken of it publicly many times. You've had dinner with each other's parents where these things are discussed. You've created bank accounts, you have paper trails and history, everyone knows the deal. Even should your partner go nuts, everyone knows of the agreement.
That's where I got tripped up. The entirety of our social environment is only a minuscule fraction of the humans in the area. In the ancestral environment, if everyone both of you knows is aware of a thing, that's the entire world for all practical purposes. In the modern environment, that's no one. Unless your social circle includes the judges and lawyers that will be presiding over the court case, none of that matters.
Naively, one thinks "Look, everyone knows the score. We can go and explain it to any Judge. They are impartial arbiters, set by society to maintain justice and fairness. All we need do is explain the situation and they'll do their best to bring about an equitable resolution."
One is wrong to think that. They are sentinels set to keep society as stable as possible and the status quo as untouched as possible. There is already a standard solution to your problem, and it will be imposed, and none of your arguments really matter. Do you really want to argue about why YOUR situation is different and unique and special, and explain why the standard formula is unjust and inequitable, given the arrangement you had that EVERYONE knows about? Really? Ok, fine, you can do that. You'll have to put off the resolution for months (at least) while court dates are made, motions are filed, and so forth. I hope you weren't trying to get on with your life in that time, because that certainly won't happen. Your lawyer bills will be in the thousands per month, so you're looking at a minimum of $10,000 just to present your case, and very likely much more.
And all this buys you is a chance for the judge to say "Eh, this is very unusual, but you make a good case. I'll adjust the standard formula by 20%." Not "Here's a Fair Judgement based on The Case At Hand." Just an adjustment of the standard resolution. Unless you were talking huge numbers in the initial case, that adjustment to the standard solution may not be worth all the time, money, and psychological turmoil you paid to get there.
This is because the court has a vested interest in NOT MAKING EXCEPTIONS. And when they do, only slightly deviating from the norm. Their goals are to keep things as steady and predictable as possible, and make sure everything cleaves as close to the Standard Resolution as possible. Simply by presenting your case to the court for consideration you are making yourself its enemy. Stop trying to rock the damn boat, it's got important places to go.
But you know what completely short-circuits the standard formula? What nips the entire process in the bud and smothers this unholy abomination in its legal crib before it can grow into the vile abortion of justice it wants to be?
A written agreement, signed by all parties.
Because two adults can agree to most anything, as long as it's not unconscionable or illegal. And once they've agreed to it and there's written proof of that, that supersedes the default procedures.
Sure, you can still fight over the details. But at least what was *supposed* to happen is documented. The goals that were originally being pursued and invested in. That paper defines the entire battleground. Without it, you are in hostile territory, and the powers that rule it just want you out of their hair.
Put It Into Writing. You don't need a lawyer. You don't need anything super official. Sure, those things help, especially if it gets ugly. But even a simple print-out of intentions and expectations, signed by both people, does WONDERS to define the territory. Always define the territory.
No one who actually cares about you will ask you to risk jumping into hostile territory. And if you've found you accidentally wandered into it, stop wading deeper in. Don't go another step without a piece of paper. It's better than a map. It is the territory.