I'm gonna take it on faith that you are already working with a RealtorÂ® and are just looking for more outside advice on this one.
Your question is a little vague, so I hope I'm answering it properly. If you're asking how much over asking price you should offer to win the contract, that depends on several factors, the primary one being - are there any other offers on the house? If there are none, then I would wager that anything over list price would win the contract because the bank would be making out better than what the owner was hoping to get.
I am not a lawyer, and the following is only my opinion from previous experiences:
Now, if the owner hasn't begun negotiating with the bank yet, that's a whole other kettle of fish and will be a drawn out process with the bank, but keep in mind that, even though it is a short sale, once you have a ratified contract - that means both you and the OWNER, have signed all contracts - then no one else can legally come in and put a contract on it and the owner cannot seek other contracts - even if the bank hasn't approved the short sale yet. The bank has to approve the short sale for the owner because of their mortgage on the house, but the bank does not own the house. AND, even if the bank doesn't approve the short sale, the owner can technically still sell the house in a short sale, but will still owe the bank the rest of the mortgage they took out. The bank only takes possession of a house in a foreclosure and that is the only time they have a true say-so in the sale of a home.