Employment Law Questions? Ask an Employment Lawyer.
If he is knowingly stating things that are false about you (or that he knew should have been false), and it was said in the presence of a third party and it has caused you or could cause you damages, then you have a cause of action against him for defamation. Defamation is the act of hurting another's reputation through false accusations or comments via verbal discourse.
Based on what you have shared with me, you do likely have a cause of action if what he said was false. Truth is the defense to a claim of defamation. That said, it is optimal if you can get the person or persons in the room to whom he was speaking to testify on your behalf, so that this is not a "he said, she said" type affair which are rarely successful in court.
To that end, if you could get them to sign written statements and date them now, even before litigation is filed, that would greatly help you case.
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Just checking in to see if you had any additional questions. I want you to be as comfortable as possible as you move forward. Thanks!