How accurate is this home appraisal?

My son is buying a home that the owner owes 271,000.00 it is appraising at $180,000 now due to the declining market. What will normally happen is once the actual appraisal is done, if it comes back at $100,000.00 the seller will drop the price for you. They know they can't sell for more than appraised value unless the buyer is willing & has the additional cash.