Good afternoon Cremine,
The short answer to your question is, "YES. The Seller can cancel after they have accepted your Offer."
A short sale is a different kind of transaction than the normal real estate purchase. You are essentially going to wait while the Seller negotiates with their Lender(s) for approval to sell you the house at a price lower than what is owed to the Lender(s). This process can be lengthy, even stretching out to many months before you have a final answer.
You'll need two things during that time:
1. Patience. While you've submitted all your documents to your Lender for your mortgage approval, and you keep your documents current and updated, you'll grow frustrated with the lack of any communication from the Seller about the status of the short sale. This is common, both the poor communication and the Buyer's frustration. Prepare for it.
2. Maintain your credit scores. Be sure not to make any dramatic changes to your credit report that could affect your credit scores. Pay all bills on time. Don't CLOSE any accounts. Don't open any new accounts. Don't run up your outstanding balances to max your revolving debt.
Once the short sale is approved, you may find the price you offered is not acceptable to the Seller's Lender. They may come back and counter-offer your price, so you will have to reconsider at that time if you wish to pay more than your original offered price.
The three rules of real estate: Location, Location, Location.
The three rules of short sales: Patience, Patience, Patience.
I hope that helps!