Buyers who are pre-qualified or pre-approved or received a loan commitment letter from a mortgage lender are one step ahead, but it's not a done deal yet. The mortgage lender will want an official appraisal of the property, and problems may arise if the appraised value is below the sale price.For example, a buyer purchasing a home for $200,000 and planning to put 20 percent down would apply for a $160,000 mortgage. But if an appraisal shows the home is worth only $180,000, the lender may offer the buyer a loan of only $144,000 (80 percent of the appraised value). That's $16,000 additional the buyer will have come up with out of pocket to meet the seller's asking price.If an appraisal comes back low and the buyers can't afford the additional cost, they may need to try renegotiating the sale price, request a second appraisal, or call off the deal. Some lenders may allow buyers to put less than 20 percent down if they purchase private mortgage insurance.