Do I Have Case Against Lies Told In The Work Place That Led To Reverse Discrimination From A Customer?

This is what’s going on with my wife right now. It’ll be easier to explain from her point of view, so I’m just going to present the situation as if I’m her…it’ll eliminate alot of confusion…
An AA male customer in my store (Large, mall department store seen everywhere) lied to his wife, whom works there, and told her that I said, ‘He couldn’t afford the sunglasses he was looking at’. Both of my head store managers believed her secondhand account, over my account of the events, without even giving me an opportunity to tell my side of things. They both screamed at me, ‘HOW COULD I DO SUCH A THING!’ I had no idea what they were talking about for at least 5 minutes. Never gave me an opportunity to speak. One of them just kept his arms crossed, lips pursed, and stared at the wall the entire time. Every now and then he’d holler at me something off subject about how, ‘We hear all kinds of things! How can we believe you!’ I asked them three times, ‘Did they check the security camera footage’ today, but they never did. I don’t think they record the camera’s…Is it legal for my employer to assume wrong on my part, because some random AA customer either heard voices in his head, or was trying to create trouble with white employees? I still have my job, as of tonight, but they were trying VERY hard to fire me. I did NO wrong, and NEVER said those things. It’s his word, against mine, and the store has no footage of the event that never took place. Do I have grounds for a lawsuit? Everyone in the store heard about me being ‘racist’ before I was even called to the office. Several of my coworkers later told me they knew about it beforehand, and, ‘What was my problem with black people?!’.
Isn’t this ‘defamation of character’, ‘discrimination’, and possibly several other offenses. I’m heartbroken….I can’t BELIEVE everyone just assumed he was telling the truth. I feel so wronged…is there someone to contact about this? Do you think I have any cause for a case against my employer? I was being looked at for training to be a manger last week, and I told them, ‘Yes, I’d be VERY interested in moving up into management.’. Now, I feel like that promotion just went out the window, no matter if they fire me, or keep me on. Is THAT grounds for some sort of suit against my employer, if I don’t get the promotion because of a racist conversation that never happened?
Thank you on behalf of my wife for any answers you may have…I’m furious that she’s being treated this way!
DP