I once played a (somewhat crazy) game of Monopoly where there were too many players, so not enough properties to go around. I ended up with close to zero property but lots of cash. So I made an "investment" in another player. She had great properties (the dark greens) but very little cash; I gave her a couple thousand dollars to build with the agreement that I would get a certain percentage of all income from those properties (I believe it was a 50/50 split) and I would not need to pay if I landed on one of the properties in question. Is this a legal deal? We considered it a form of trading -- I gave her money and in return I would get money in the future.
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While you are technically allowed to make such a deal, due to the fact that trading allows you to give another player money; there is nothing in the rules that would enforce your opponent to keep her end of the deal. If you later landed on the property in question, she would be fully within her rights within the rules to ask you to pay, and you would have to pay according to the rules. Similarly, if she makes money from someone else on that property, there is no rule that requires her to give you part of that money.
So as long as you fully trust her to do what she says in the future, you would be allowed within the rules to do this.
From the Official Monopoly Rules by Hasbro (my emphasis):
Any exchange of money for future considerations is a form of loan, so by the official monopoly rules your actions are illegal. One is always allowed to invent a new game based on an existing one, through the use of common agreement on house rules, but one cannot then meaningfully call it by the name Monopoly.