You should inquire with the listing agent as to where in the process the short sale is. As a real estate attorney that handles short sales, I begin my negotiation process as soon as the listing comes in the door. So by the time an offer comes in, I have put together an entire financial package on the Seller (3 month bank statements, hardship letter, 60 days paystubs, etc.) This means that I can then present the offer to the short sale lender and things will go much smoother and quicker.
Unfortunately, this is not the case when the Realtor or attorney handling the transaction on the Seller side is not familiar with how a short sale works. They may begin the process at the time an offer comes in. That means that they won't have an approval letter, if one will be forthcoming, until several months, at least.
Since short sales are never guaranteed to go through, you may want to consider an attorney or firm who only charges if the deal actually closes. This is how our firm bills. We don't charge if the deal falls through. However, if you had conducted an inspection and or appraisal, you may still be out that money.