Right now there's nothing that would differentiate Monopoly money from any other game object you can have, since currently negative amounts of Monopoly money can't happen. So, you can't spend Monopoly money that you don't have. Otherwise, this would be equivalent to, say, constructing a building using logs you don't have and going into negative logs. Just because the rules don't state you can't do something doesn't mean you can automatically do it. However, if Proposal #145 is accepted, you then may have a case (since that proposal does explicitly allows negative Monopoly money to happen).
Also, Proposal #148 is illegal. Your proposal cannot just override rule IX-3, since the rules are in enforcement, and your proposal is still just a proposal. The very act of making that proposal is illegal.