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