Answer

Several later authorities (Acharonim) discuss this. In my opinion there is no obligation at all, although it is a virtue. The permission/obligation to hate refers to wicked people, not to just anyone who hurt you. The prohibition against hating in one's heart, at least according to Maimonides, only forbids keeping the hatred inside your heart rather than hatred itself. Although the halakhic decisors disagree about this. Clearly it is preferable not to hate, and if you have something in your heart against him, tell him.