First, this is not legal advice. For legal advice talk with an attorney.
Second, I realize that you are frustrated. Without knowing all the details, seeing the paperwork, etc. I am not sure that this is the best place for you to get the advice that you need.
Third, the professional that is handling the purchase for you should be the primary provider of advice. That is their role.
So, in a brief summary:
Sometimes lenders involved in short sales have buyers sign an adendum. Did you? If so, what does it say? In many cases the forms stipulates what might happen and what your recourse might be.
Also, did you read the CAR Shortsale advisory?
One of the big problems in purchasing a short sale is exactly what you encountered here...invest time, money and effort, then get zero.
It should be noted that (disclaimer - generally speaking) if a seller back at after contingencies have been removed, and they change their mind, the buyer can sue for "specific performance". The seller needs to sell, or pay the buyer damages. Just because the sale is a "short sale" subject to lender approval, the seller still is the owner, meaning that they are liable to follow the purchase agreement.
I suggest that you talk with your Realtor, if they are not helpful, talk with their broker. If they are not helpful, talk with an attorney.
Sorry for your situation. Good luck.