What is the expiation for someone who intended to harm another person, then recanted their intention and repented, knowing that the person did not suffer any harm?

If the questioner retracts his decision and does not carry out what he intended of harm, then he is not held accountable for the mere intention. Rather, a good deed is written for him for abandoning that for the sake of Allah. However, if he carried out some of what he intended and harm occurred, then he must enable the wronged person to exact retribution or seek his forgiveness. If that is not possible, then he must humble himself before Allah, seek forgiveness for the wronged person, and increase in good deeds. And if he fears an increase in discord by seeking forgiveness, then he must turn back to Allah with supplication so that He may make his adversary pleased with him.