If you are serious about a "short sale" an it is a pre-approved sale price, meaning the bank has all ready approved this price for selling the property, it is important to try to make your offer close to the asking price.
To qualify as a short sale the home owner has to demonstrate "financial hardship" and to have the property pre-approved the owners and bank have reached agreement on the sale price. This means the bank has agreed to accept less money than the owner borrowed to satisfy the sale of the home.
Banks have a history of when being put in the position of discounting the property even more they will often let it go to "foreclosure." This most likely will not result in you getting the home.