Is it safe to assume I am not under investigation?

Trust me, you've done nothing wrong. As long as what you posted here was entirely truthful, you have nothing to worry about. As long as you can get another job with good references, then its no problem. And if the police ARE called (which I doubt, at a store that busy, $50 is rather nominal), tell them what you said here. You did what you thought was right, which probably WAS right, and they have no proof saying otherwise.