What should I do about my complaint against the postman whom I thought had acted improperly, but then his innocence became clear, especially since I am unable to contact him to ask for forgiveness, and he might not be a Muslim?

What you did was wrong towards the worker, and you should have verified the matter before complaining. You must rectify the injustice he suffered because of you. If you do this, we hope that no sin will befall you, because your action was a mistake, and you exerted your effort to remedy and correct the situation.