So check with your attorney on the terms of contract you both signed on. Normally your attorney could make a call to buyers attorney to convince in time extention of the contract.
May be it works.
It is a tough thing to deal with but it is just the market right now. If the tax credit is not extended past April, you will most likely have a better chance to find a buyer that is willing to go the long haul on the short sale approval. It is still a tough process for a buyer to swallow when they want to buy a home and move in...waiting weeks, even months to get the approval is just hard for anyone that is buying a home.