First and foremost, you are asking a LEGAL question that for you to make a fully informed decision about you should be discussing with a Real Estate attorney in Florida. If you have a buyers agent, they should be able to refer you to a lawyer that might give you an overview of your options and the possible outcomes.
That being said, by virtue of being in the business and understanding the logic behind things, it is usually communication or lack of communication and information that parties have that make them make the decisions they make. The issue the seller is having with the FHA appraisal and the fear is that the appraisal will stay with the house for any future FHA loans for 6 months. If it comes in low it virtually eliminates all FHa buyers, but then you have the issue that even conventional loans that are being sold need to meet many of the same guidelines as government backed loans if they are going to be sold on the aftermarket. At some point an appraisal is going to need to be done for the seller to sell his home. Any appraisal can come in low in our current market. If all parties feel the appraisal is low for the actual market, it can be reviewed and contested with solid documentation if all parties are willing to work through the process.
Ideally your purchase agreement should have indicated the type of fiancning prior to the seller accepting the offer. If not, while I am not a lawyer, it does potentially "change" the terms of the agreement and any time there is a change in the terms all parties must agree.
Also, in general terms, it is very difficult to force a seller to sell their home or to force an unwilling buyer to purchase. Yes, these are contracts and in consulting an attorney you will learn your options, but it will most likely come down to how badly you want the house to go through the time and expense of a lawsuit not knowing the outcome.....which could just be monetary damages???? Again consult an attorney, but look at the big picture and have some GOOD COMMUNICATION with your agent who will hopefully have GOOD COMMUNICATION with the sellers agent who will then have GOOD COMMUNICATION with the seller.
Any real estate transaction is fraught with high emotions as this os often the largest financial decision both buyers and sellers make at any given time. It is even a greater stress in our current market where many selers are losing equity which often equates to retirement funds.
Good luck! and COMMUNICATE!