(CNN Money; New York) During the three months ended Sept. 30, short sales in which homeowners had fallen behind on mortgage payments soared 22% over last year, according to a report released Thursday by online marketing company RealtyTrac. By comparison, short sales by people current on their payments went up 17%.

In a short sale, homeowners sell at a price that is less than what they owe the bank, and the bank agrees to absorb the loss. The bank unloads the house and the homeowner gets out of a mortgage he can't afford.

And currently, homeowners don't have to pay federal tax on the unpaid mortgage debt because of a bailout-era law known as the federal Mortgage Debt Forgiveness Act.

But the act expires on Dec. 31 and, unless it is extended, the IRS in January will start treating unpaid mortgage debt as taxable income for many borrowers. The average amount of forgiven debt in a short sale is about $95,000, according to Blomquist. The tax on that could go as high as $33,250, even more if the Bush tax cuts expire.