1) Yom Kippur does not atone for those sins which one has committed between himself and his friend unless he appeases his friend. It follows that every person must check if he has other people’s money in his ownership and return it. Even if he only sinned against his friend with words alone, he must appease him, either by asking himself or by sending a messenger. (Kitzur Shulchan Aruch 131)
2) The person from whom he is asking forgiveness, should forgive wholeheartedly, and not be cruel and not forgive, as this is not a trait of a member of Klal Yisrael. Even if he has given him much pain, he should not take revenge or bear a grudge. One who does not remove hatred from his heart on Yom Kippur, will not have his prayers listened to. Everyone who overcomes his traits and forgives will have all of his sins forgiven. (ibid)
