We have a new FAR/BAR (Florida Assoc. Realtors/BAR) approved form for use with all Short -sales. It limits the amount you are bound to the offer to 45 days max., at which point you can request your escrow be returned and walk away.
Your agent should have included the form in the package. Move on to another property, you can probably find a Foreclosure (which gives you a responce in less than 7 days) with the same or better options for the same price.
Are you in a hurry to lose money?
I'm not sure if you heard, but housing prices are going down, probably for the next few years at the least.