Suppose, person A fires person B because B behaved unethically and ignored the company's code of conduct. A, however, did not have enough evidence. He later did find evidence, but that doesn't justify his initial course of action, that is, firing B on the basis of no evidence.
You could call this being "right for the wrong reasons." It's not precisely a fallacy, but it does mirror the fact that an argument cannot be considered sound (or cogent) unless it is first valid (or strong). The fact of a right conclusion has no bearing on whether the argument itself is good or bad.
It's not a fallacy at all. A fallacy is relying on poor reasoning.
A fired B without having enough evidence to fire him. Later you say A fired B with no evidence. I'd ask you to make up your mind there. Let's assume there was evidence, but not enough evidence.
It is also not clear what "enough evidence to fire him" means. Depending on the country where this happens, firing someone "without enough evidence" may or may not have legal consequences. On the other hand, keeping employing someone who is suspected of behaving unethical might also have dire legal consequences. If you think an accountant is cooking the books but you have not enough evidence to prove it, firing him might still be the right thing to do.
Anyway, we can for arguments sake assume that someone made a decision which based on the known information was wrong. We have no reason to believe there was any fallacy involved in the decision. A fallacy might be something "Joe's accountant took a day off, and then was found stealing lots of money. My accountant just took a day off, so he will probably steal money as well". That's poor reasoning; there is no reason to think that taking time off and stealing money is related. But this didn't happen, no fallacy.
Obviously after a decision is made, often more information comes to light, which might have changed or confirmed our decision had we known it earlier, or which might have changed a decision from being right to wrong or vice versa. This happened here. But that is after the fact, the decision when it was made was wrong.
Now I could think of some fallacies that could trap people in this situation, but you didn't mention any of those in your question.
Choosing a course of action without a complete knowledge of all possible evidence is not a fallacy, it is a common fact of life. If person A fired person B because of perceived ethical misconduct then it is possibly a well thought out decision on the part of person A because they do not want to risk the negative impact of person B's conduct. It is simply a risk assessment, and logically valid at that.