there is no assurance. if they already have a visa number, meaning that your visa is issued, why the heck you have to leave the country? There is something wrong here.. They either don't know the rules (which means they are unprofessional), or they dont want to pay 500 for switching status of your visa from family visit to work..
Oh, and the reason that they do not want to hire applicants locally has no sense. As if they want to hire you "overseas", the first thing they must do is pay for your ticket..
How you can get any assurance from such weird company?