Save your money on attorney fees. The short sale addendum gives EITHER party the right to cancel in writing PRIOR to the bank issuing an approval letter.
HOWEVER, you should have your agent investigate a bit more. How much are these sellers negative with their equity? Do they owe $800K & the property is worth $500K? Your best option at this point is having a 'meeting of the minds' of sorts,,,the owner needs to sit back & say "hey, how long is going to take me to break even on this house? Should I be taking advantage of the mortgage debt forgiveness act NOW before it expires at the end of 2012?
The owner needs to know that even though they missed payments (if that's the case), they could still purchase another home again, maybe even in the same neighborhood in 12mos.
Let me know if you have any questions, I can also look into this for you, by checking to see what their approx. loan balances are. Email me directly, I won't re-post on this same thread.