Lucy,from your question you appear to be a seller though you show as buyer under title. If you are buying and house under appraises whether you must buy or not depends upon the terms of contract of sale. Often attorneys will add a clause where seller will either reduce price or buyer can walk. This is not always the case. I have had contracts where even if a home underappraises by a certain amount the sale goes thru. (especially common with large down payments)
If you are seller then your listing agreement terms are what cover your duties to your agents and the contract terms for the sale will effect the sale. Under appraising alone may not be grounds for reducing your sale price. Speak with your attorney. If however it does and you become a short sale, negotiations may begin with Realtors, your lender and your buyer. Until you check with all these parties you may not know all your options. (lender could forgive some of your debt or buyer could up their down payment if needed) Buyer could also try getting another appraisal as these are opinions of value and subject to variance.