Did u sign a contract with ur employer for a certain period of time that u will work with him and he can have ur certificate till the contract is getting over? If u have signed something like that then u can do nothing.Else send a mail to His Highness directly. mentioning ur problem.
The results would be dramatic.Before that warn ur employer that u will proceed legally thru a lawyer if u have guts and money .Else obey ur employer and ask him what is needed for him if he wants to return the certificate.

rgds
srivatsan