If you're talking about a Short Sale, the addendum allows the Seller and Buyer to easily cancel under certain situations. Read the short sale addendum carefully.
If this is a regular sale then it all depends on the original contract whether or not the Seller can enter into a new contract without getting a cancellation on the first.
Ask your Realtor to contact the Florida Assn of Realtors Legal Hotline and they can answer legal questions for your Realtor. If you do not have a Realtor then you need to contact an attorney.
Hope this helps.