If you truly feel that the appraiser was off by more than 10%, then you may want to hire your own appraiser or tell the bank that you are willing to pay for another appraisal from a different source.
Folks are lazy.
As Dan hinted, if you have proper, eligible comps, hand them to the appraiser.
Of course, before you do that, take the necessary steps to insure conditions are present so it appraisers at the amount you want.
Your buyer should have been prepared for this consequence and had a pile of cash on hand to keep the deal rolling. As you know, it not about the value, it is about reselling the loan.
Not all buyers have what it takes to participate in the short sale marketplace.