This is a very interesting question.
Who is negotiating for you?

The link below shows a 30% decline in value over the past 12 months, so depending upon the length of time you bought your home, that would make sense. However, if the true value is the BPO, it will be interesting to see how the appraisal turns out.

I hope that you have a very good REaltor representing you. They are the ones to answer this question.

It will depend on how you negotiated your short sale with the bank. Are they going to come after you for the rest of the money owed on the mortgage, or did you negotiate that what ever they get is the final amount owed to them. You may have been better to sign the deed over to them in lew of foreclosure and made sure that no derogatory statements when on your credit report.

There is a good chance that your lender will approve the short sale.I am assuming that is what is happening here. If the banks BPO came in $30k less than the offer, you have a better chance of getting it approved. My main concern would be if the buyers appraisal comes in at or around the same amount of the BPO, the buyers may not get financing. As for amount to be paid back to the mortgage company-that would depend on your hardship, and if the debt forgiveness act of 2007 will be in play. Talk to your short sale negotiator/Realtor or your lender to find out the answers. Good luck!