Why don’t you just tell them the truth. Your return has just been or just about to be filed, and the IRS hasn’t processed it yet. I presume it is still standard practice for underwriters to request and receive authorization to receive copies of returns from the IRS (which it sounds like you’ve already provided). They’ll get the copy (or transcript) of the return after it’s processed.
Essentially the lender is in the exact same position as they are when a borrower has a valid extension to file their return.
