[SamBam]

As far as I can tell, they aren't explicitly disallowed. The rules do disallow lending to another player:

> MISCELLANEOUS… No player may borrow from or lend money to another player. [1]

I could see two arguments: First, board games rules generally do not need to contain a list of everything not allowed. In Risk you are not allowed to gift your tanks to another power in exchange for them not attacking you. By this logic, you're right, such deals are not allowed.

On the other hand, most players I've met treat Monopoly as "anything goes" in terms of between-player business deals, so long as they are not restricted by the rules. The fact that the rules explicitly disallow lending, but not any other deals, could point to other deals being allowed.

...I expect you're right, making deals is not allowed in the rules. But disallowing them would certainly turn a dull game into an even duller game.

1. https://www.hasbro.com/common/instruct/monins.pdf

[indrax]

Any sufficiently advanced game of monopoly is a violation of SEC rules.

[lmm]

> The fact that the rules explicitly disallow lending, but not any other deals, could point to other deals being allowed.

The rules explicitly allow particular kinds of deals and not others:

    Unimproved properties, railroads and
    utilities (but not buildings) may be sold to any player as a private
    transaction for any amount the owner can get; however, no property
    can be sold to another player if buildings are standing on any
    properties of that color-group.

[MrKristopher]

The PDF doesn't mention trading rules. You can refer to tournament rules or Monopoly computer games for what is or is not allowed -- and these types of trades are not allowed.