Your creepy "friend" probably knew what he was doing when he gave you the knife and when he left quickly. He was afraid you would give him something in return which would nullify the bad luck he wanted you to have. You could mail him a penny to make the exchange complete. He'll know what it's for.
You might be letting your imagination get the best of you. I have friends who are caterers. I have gifted them with knives and nothing bad happened.