That depends on what your contract says. If the agent of the buyer was sharp, he may have included a contingency that the appraisal must be equal to or greater than the selling price. If that contingency is in there, the buyer can simply walk, if he desires. If no such contingency is in the offer, then you have a binding contract, regardless of what the appraisal came in at. In do not know to whom you "appealed" the appraisal, but that will not work. The lender chooses the appraiser and they do not second guess the appraiser chosen.