If Congress drives the economy off the "fiscal cliff," wave goodbye to short sales that have helped the housing market get back on its feet.

At risk is a provision that erases taxes on selling a home for less than what's owed to the bank.

Expiration of the tax treatment would create a major new headache for the one in four homeowners who owe more than their house is worth. Those "underwater" sellers would have to come up with a big check for Uncle Sam to pay the tax on the difference.