The due on sale clause in today’s mortgages basically states that if you sell or transfer all or any part of your interest in the property without your lender’s prior written consent, the lender (at lender’s option) may declare you in default and require immediate payment in full of your entire mortgage balance. Even a lease option is considered transfer of interest and would trigger the due on sale or acceleration clause.
There have been recent cases in which property owners have sold their homes to a “holding company” for some paltry, insignificant sum (perhaps $10 or so) in which the holding company agrees to be responsible for the future mortgage payments. The holding company charges the owner a fee of several thousand dollars for the privilege and then fails to make the payments.
As there are few (if any) mortgages today that are freely assumable, owners in such situations are almost certainly in violation of their mortgage agreements and will face foreclosure.