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DearCustomer- Your husband could sue for fraud. Unless he has something in writing that they need this money for the down payment along with the letter then it will be his word against his daughter's that all of this is true. Obviously she could simply present the letter and state that it was a gift and was never needed for the down payment.
It sounds like his daughter in not very trustworthy so I am assuming she will lie rather than admit she owes the money. If your husband can prove that this money was obtained by fraudulent means he may have a chance of recovery. He will have to file the suit in NC, not FL since that is where the defendants reside and the property in question is located.