Is your employer based in Reunion, South Africa, or elsewhere?
I would have thought the work permit is straightforward enough for the employer to obtain if they're based in Reunion (or even mainland France) in that they need to provide French authorities with your contract.
Do be very wary of upping sticks and moving to Reunion if you don't have iron-clad confirmation, I know of someone whose employer cancelled their contract just 48 hours before they were due to move countries to settle in Reunion. They had sold their house and everything to make the move.