A 1099 simply reports income other than taxed wages. For example, if a debt of $10,000 is settled for $6000, the $4000 different would be considered income. Because it is in excess of $599, the creditor is required to issue a 1099, which will state simply the dollar amount.
Short sales are actually easier to document than regular sales. Fill out the 1099 as usual with purchase and resale price. You don't have to worry about the "taxable improvements".