The builder can't change the tax laws. As a general rule, it doesn't matter what he calls it. What matters is the actual substance of the transaction.
I suggest that you consult a tax lawyer (not accountant). If the lawyer says you can deduct it as mortgage interest, ask for a written opinion so that you have solid backup if the IRS questions it. And keep in mind that if you deduct it now, you cannot include it in your basis. It's one or the other.
