Question: Are side deals allowed between players?
Answer: It depends on your definition of "side deals."
A player can cut a deal with another player to buy or sell a property for cash, or to trade various properties, for example. You can do this on your turn or between the turns of other players. Deals must include the exchange of tangible items such as cash, properties, and Get Out of Jail Free cards.
In official tournament rules, you can never cut a deal involving future considerations, such as a promise to give someone the $200 you earn the next time you pass Go.