This is really a question for a real estate attorney, as we are forbidden by law to give legal advice, but I will say that I have NEVER been in a closing where the lender did not ask the buyer and seller to sign a form that stated that there were no side deals or loans or money given that was not disclosed on the HUD as part of the deal. So if you sign that and do not disclose, common since would tell me that you are committing fraud.
I would seek the advice of an attorney. It sounds like the builder is doing a seller carry-back for a second mortgage to help you purchase the home - but all this must be disclosed to the bank to see if they allow that to happen and how it changes your ability to buy the home since now your payment will be higher.
Good luck. - Mon Sep 8 2008, 14:31