Legal Question in Real Estate Law in Florida

Odds are you can't. If you sell it subject to a present 1st mortgage that the Buyer isn't assuming, and you don't pay off that mortgage, you probably have violated the terms of the 1st mortgage Due on Sale Clause. If the bank finds out, they can foreclose.

Of course, if the bank keeps getting paid, these days they don't go looking for problems so it may not come back to haunt. But the title policy to the Buyer will have to have an exception for the situation, and some title companies may not do the closing for you.