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.
From the Official Monopoly Rules by Hasbro (my emphasis):
Money can be loaned to a player only by the Bank and then only by mortgaging property. No player may borrow from or lend money to another player
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.
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.
This is part of the reason why board games have a limited set of tokens that goes along with the number of players on the box - the mechanics of many games break down when the number of players gets too high, and monopoly is no exception.
As you are already house-ruling to allow more players than intended into the game, it would be possible for you to house rule things like shared ownership in property (each player collects a portion of rent when anyone outside the group, and may own their partners a share of rent if they land on it) or other group strategies, but these would be house rules and would be against the rules of monopoly as written.
As written, the only ways for players to interact are:
- Collection of rents.
- Trading of properties for other properties and/or cash.
- Game held auctions when unowned properties are landed on but not bought by that player.
Any other kind of interaction between players is expressly forbidden by the game, which does not allow any kind of loans, and giving something now, in this case money, for something in the future is the definition of a loan.
The only kind of loan allowed by the strict rules of monopoly is a mortgage, a loan from the bank during which the mortgaged property collects no rent.
When talking about house rules, anything is legal, as long as it's agreed to by the players beforehand, when talking about printed rules, what you did is very much illegal in monopoly.