Question: When I mortgage a property, what can I use the money for?
Answer: Anything you want to, so long as it's legal under the rules of Monopoly.
The only restriction in this regard is that a player cannot pre-mortgage a property to finance its own purchase. For example, say a player wants to purchase Boardwalk but can't do it with his or her current assets. That player cannot say, "I'm going to buy Boardwalk by mortgaging it, and then using the money I get for the mortgage to complete the purchase." You must own a property before you can mortgage it.
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