[strike]I’ll assume two things. First, it was absolutely clear to everyone that “being promoted” meant getting a raise. Second, the manager said clearly that you were promoted.
Even so, you’d have a near-impossible burden proving it. You entered an oral contract. Because there’s no written evidence, and probably no testimony from your old boss, the “course of performance” under the contract would be the most persuasive evidence of what the contract entailed. You went six months before bringing it up. That suggests that 1) the contract never happened or was not made sincerely, or 2) you released your employer from the contract.
Second, in most states, unless your employment contract makes clear that your employment is not at-will, you can be fired or demoted at any time. You don’t have any “right” to your position or hourly rate. Even if your old boss promoted you, your new boss was free to tell you to take minimum wage or be fired.
Now, if you spent six months doing the work of someone with a higher position, expecting to be paid for it because everyone else who did that work was paid at a high rate, you may have a claim. Then there’s compelling evidence that the promotion happened, and you deserve the “benefit of the bargain.” But you’ve spent six months doing your old work, and meanwhile you’ve been paid at your old rate. You’ve hardly been treated inequitably.
I’d let it go and move on.[/strike]
After reading your second post more carefully, I think you may have a legitimate claim. It sounds like you were a supervisor of sorts. Was there a standard rate at which supervisors were paid? If so, what was it? What were you paid? Can others testify to you doing supervisor work? Can you get in touch with the old manager who agreed that you would be paid the higher rate? Did you ever mention to anyone that you expected to be paid the higher rate? Did anyone besides your old boss ever mention it? Is there any form of written contract or employment manual that mentions salary?
I can’t advise you of course, since I’m not an admitted lawyer and don’t know employment law, or even what state you’re in. But if you can find a lawyer who would take the case on a contingency basis (he fronts the costs and takes part of the winnings), you may as well try it.