The prior agreement vs measure of risk is not a mutually exclusive point.
What you’re glossing over is the nature of that agreement, and what you’ll find is the it spells out,
wait for it
wait for it
how risk is shared!
From your OP Allison and Sarah could have had a prior agreement, that would have spelled out how risk is shared. Allison putting up cash, Sarah investing talent. Both get a portion of the proceeds.
From my OP: the dishwasher and the owner have a prior agreement, show up and wash dishes, earn $10 an hour. The dishwasher chooses to avoid risk in exchange for security. No not job security that’s absurd, but guaranteed return on investment*. The dishwasher puts in an hour of investment, and gets $10 in return. The owner doesn’t get that guarantee. She might put in 80 hour weeks for a year, only to see her business fail. She also might put in 20 hours a week and hit the jackpot.
Go back to the casino example: If I put down $20, I stand to win or lose. If I pay someone $5 to make the bet for me, we have an agreement to share the risk in that he is guaranteed $5, but not a penny more. His level of risk is perfectly matched by his reward, which is completely independent on my level of risk.
There is no reason we couldn’t have a prior agreement to share the risk. It could be a shitty agreement like, “I’ll pay you $1 to make the bit, if it wins you get a $4 bonus.” Consider that for a money in the context of a casino. If you think I’m skilled and know where to place the chips, you might opt to share in the risk. Now your real risk is that you could choose to work with another gambler that could be more or less successful.
You’ll notice that my broker wants a $5 commission, regardless of whether or not my trade is successful. If my stock takes off tomorrow, there is no way in hell I’m giving him more than $5 when I sell. He gets the $5 off all my failed trades, and that’s his choice. He can’t have it both ways. Either he takes a loss with me (perhaps zero commissions on losing trades) or he forgoes my winnings (getting just $5 regardless of what I earn).
That’s how the prior agreements work, they spell out how each individual will be compensated for their level of risk. And what you’ll see is that it lines up pretty fairly such that those who are guaranteed something get less of the profits than those who are not guaranteed.
*assuming solvency