At the very beginning of the game, offer your opponent $1,600 in return for a 50% stake in all the rents they collect for the rest of the game.
Your opponent accepts, and you can't pay. Game over.
> 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.
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.
This can happen after the first player rolled a double, so the first player’s turn doesn’t even have to be over for a player to go bankrupt.