This can be referred to as disputing an appraisal as well. When an appraisal comes back below the agreed upon price in the contract, more often than not the seller is not going to want to just drop the price. There is the option of challenging the appraisal.

We've done this successfully on a few occasions. It doesn't happen to us often as listing agents because we price properties accurately. If a seller wants to list at a very high price, we will not take the listing unless we are confident it can appraise. The best we've done on challenging so far was getting an appraisal that came in at $235,000 to go up to $282,000. We took a look at the appraisal and what was done, then took another approach to show the appraiser how we came up with value. 

Because we knew the rules, and what could be done, we were able to make it happen. The appraiser was not doing anything wrong, but for the value to come in the way it did we as the Realtors had to suggest a different direction. It wasn't easy, and it took some time, but it worked out.

In challenging an appraisal, it could be an adjustment, choice of comps, location of comps, etc. The important thing to remember is this isn't you/your agent vs. the appraiser. Things should always be approached as helping each other come to a common understanding of what the value of the home actually is. Appraisers are human and mistakes can be made. I will say that appraisers are right most of the time.