I agree with Army Judge. This may be more a matter of what's in your lease than any law.
If you signed a contract that says you use union labor then that's what you agreed to do and you are bound by it.
If your lease doesn't require that you use union labor you don't have to.
However, if your tenant improvements require the landlord's approval and he is withholding approval contingent on you agreeing to use union labor that's probably something you can take to court and win on.
Consult an attorney and have your lease and situation reviewed. After all, you are a "businessman" and businessmen should have lawyers available to give them legal advice and not rely on strangers on the internet.