1) What you describe would be considered fraud on the lender: converting a gift used to qualify for a mortgage to a loan would be seen as showing that the "gift" was never a gift, but was always a loan--simply that you lied about that fact. If this somehow came to the lender's attention, they could take legal action, including potentially rescinding the loan and requiring its immediate repayment.2) The note would not be enforceable. To be enforceable, a promissory note must meet the criteria to be enforceable as a contract; that requires that each side give the other consideration, or something of value. But your son already has the money; you are not giving him anything in exchange for his promise to pay; therefore, there is no consideration and the note would not be enforceable. If you son chose to not honor it, there is no way you could enforce it.