from Home Buyer’s FAQ on yourvalleyproperty.com
What happens when the appraisal does not support the purchase price?
Surprisingly you have more options then you might think. This is not necessarily a deal-breaker. You will have the choice of walking away, as a satisfactory appraisal is a condition of your contract. Renegotiating the price or terms is often a viable alternative. You may even have discretion to continue with the agreement “as is” depending on what percentage of the purchase price is covered by your down payment.