Since 2007, homeowners whose banks have forgiven unpaid mortgage debt after a short sale, principal reduction, or foreclosure have not been required to count that money as income on their tax returns. But the end of the federal Mortgage Forgiveness Debt Relief Act will mean that borrowers, who have been spared tens of thousands of dollars depending on the amount forgiven and their tax bracket, may be faced with very large IRS bills after losing their home. Bottom line: unless lawmakers extend the Act, the IRS in January will treat any unpaid mortgage debt as taxable income for many borrowers.