In a Short Sale, why does the bank need to sign off?

In a short sale, the seller’s bank may or may not choose to release the seller from any deficiency owed after the short sale is complete.  The seller, therefore, must come to a written agreement with the bank that after selling the house the bank will not be able to pursue the seller for the deficiency.  It is vital for a short seller to seek independent and competent counsel to verify that his interests are protected in the paperwork against the bank.