Great answers already. Everything is negotiable...depending!
How is a short sale different?
In every way! There exists an illusion of rules but the reality is anything goes. In the "Wild, Wild, West of Real Estate' also known as short sales, the buyer is in are the most excruciating roller coaster of a ride imaginable. And, as the buyer, you are in the most insignificant, least influential position of all.
Yes, you and the home owner can agree on a price and sign a contract. That's the easy and mostly irrelevant part.
Next the bank, (or servicer) and investor, and PMI company and second lien holder must ALL agree to the price for which they will let you buy the home. It may be considerably higher than listed price depending on the strategy of the listing organization. So you finally agree to the new negotiated price. If any of the parties involved believe the owner has any money hidden under the carpet there will be a gap in the agreement compelling the owner to come to the table with cash. Not a good indicator for success.
Then the bank presents the final agreement to the owner. This may involve a 1099, unearned income for the deficient amount. If $100k was the write off, the owner can be facing a $30K IRS bill! The owner decides who they want to fight, the bank or the IRS. (every situation is different) After 3, 6, 9 months or more, the deal crashes, all time and effort wasted. Only 30% of listed short sales in the Tampa area close. Short sales are NOT deeply discounted from traditional sales as area statistics reflect.
It's certainly doable. But, it is incredibly important you have a great reserve of patience and trust in the professionals representing you.
Best of success.
Other than that, short sales are great!