Florida Governor Charlie Crist will make the appointment, and since he himself is already running for the seat next year, he is expected to name someone who will not run for a six-year term.
If that’s the (unenforceable) condition on the appointment, and if Crist would offer it to Diaz-Balart, then Diaz-Balart would seem to have two options. One would be to accept the appointment and end his Congressional career in the Senate. The other would be to accept it, serve in the Senate, and run next year to return to his House seat. Florida law would require a special election to fill Diaz-Balart’s seat in District 21.