Everything comes down to one issue: who is responsible for what. You and the seller signed the contract, the bank did not. They can ask for $30,000, they can change their mind and simply decide not to release the title at all. Nothing is binding for them!
This is a risk one has to take with short sales, but the owner (seller) should be the one handling that, not you.
Therefore, you can add $30000 to your sale price, and let the seller work it out with the 2nd bank. I am surprised that the agent (though a dual agent) asked you to do it!