"Investing" in other players in Monopoly
From the Official Monopoly Rules by Hasbro (my emphasis):


  MISCELLANEOUS:
  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.