Standard MN Realtor Purchase Agreements do not have provisions to permit such a scenario. Some banks do require custom forms or addenda that can override the PA and allow the bank to consider and accept a different offer even after final acceptance. Often this is called a unilateral cancellation provision. Each bank seems to have their own paperwork so your mileage will vary.
There is a difference between the ability to accept different offers with unilateral cancellation and not performing on an accepted offer. In theory, the seller could be sued for specific performance for failure to consumate a transaction, however this could be overridden by some of the banks' addenda.
Most often though in a short sale situation, the standard MN forms are the only forms used. If the sellers are actively working to sell the home (which they are if they signed a listing contract and a purchase agreement) then it is usually safe to assume that they will perform as agreed.
As with any contract, there has to be faith by both parties that the other party will abide by the terms... I've heard of very few situations where a buyer or seller has litigated over a failure to perform.