You have no idea what the bank is owed. The only thing that matters is
1. How much is the property wroth.
2. How much it is worth to you.
Your offer wasn't accepted. They can legally offer whatever they like. You don't have to accept there's. Feel free to offer a different amount if you wish or walk.
If the offer is for as much as the outstanding debt secured by the property, it ceases to be a short sale anyhow. The current owner doesn't need the bank's approval.

