Real Estate Law
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I don't think you're being told correctly. If you incurred the federal tax lien before you take out this loan, it is going to be a first lien on any property you own or acquire. The second you buy the house, the federal tax lien is going to attach to that property, and because it was filed first, it's going to have priority over the mortgage.
I don't think the mortgage company will go through with the loan until the lien is satisfied or subordinated.