Based on the number of condo sales in your community, and the value those solds represent, carefully choose how you respond to this buyer. To a lesser degree, consider the other options the buyer has regarding competing condos.
Consult your real estate professional to provide inventory turnover and activity statistics.
I agree with Chris a short sale may be the only option for you here. This is something that you should discuss with your agent and make sure that they are competent to handle that kind of transaction.
Here's a starting point: