The bank can not breach a short sale contract because the bank is not a party in the contract.
The contract is between the seller and the buyer, subject to 3rd party approval, rejection or counter (3rd party being the short sale bank(s)).
The bank can come back dictating new terms, like a countered price or timing of the closing.
Because each counter kills the previous offer/agreement, you don't have to accept new, bank countered terms. If you don't accept these terms, you can terminate your contract.
However, if you do accept these terms, you have to obey by them.
Hope this helps,
Beachfront Realty, Inc.