http://en.wikipedia.org/wiki/Short_sale_(real_estate)
"A short sale typically is executed to prevent a home foreclosure, but the decision to proceed with a short sale is predicated on the most economic way for the bank to recover the amount owed on the property. Often a bank will allow a short sale if they believe that it will result in a smaller financial loss than foreclosing as there are carrying costs that are associated with a foreclosure."