Is expiation incumbent upon someone who swore not to hit his friend who had ruined something for him, but then the friend denied his action, so he slapped him in a fit of anger?

It appears you have broken your oath because you swore not to hit the person, and then you hit him. Therefore, an expiation for breaking an oath is incumbent upon you if you swore by Allah or one of His attributes. A person who is angry is accountable for his actions, unless his anger is so severe that he does not comprehend what he is saying.