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Technically, it could be considered mail fraud. In order to prove mail fraud you have to prove that at the time she made this agreement with you that she never intended for the money to be used on the down payment. This intent is tough to prove and you did send the money voluntarily, although you now find out it was used for something else. What the US Attorney would look at is some way to prove her intent at the time this deal was made and many times in a boyfriend girlfriend situation (not an internet romance) the US Attorney will not pursue criminal
charges. This is a tough case for you to convince the US Attorney to take charges on, but you can indeed report it to the US Postal Inspector or the FBI and it is up to them to put the case together and present it to the US Attorney.
It will be a breach of contract with her and it would be grounds to file a civil suit against her to recover your money by payment or by you being able to seize property that she owns or garnish her wages.
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