Fannie Mae wont postpone Trustee Sale Date

Had a listing in a great neighborhood and finally got short sale approval on it.  The bank gave us until August 26th to close escrow or the foreclosure sale date is set for August 30th.  The cash buyer walked and Fannie Mae will not postpone the sale date.  We need an extension to get a financed offer in.  This property will sell..its in great condition and its move in ready.  Its in a beautiful neighborhood and next to great schools.  The buyer walked at the last minute.  Its so hard to understand why these banks wont work with us and postpone the trustee sale date...just so frustrating when you do everything you can to help the sellers and the banks just dont seem like they want to help.  Any suggestions?