One who wrongs his friend should make up with him as soon as possible, as the longer the friend stays hurt, the greater the sin. Nevertheless, if one did not do so because he was negligent, embarrassed, or unable to placate his friend, he must make a special effort to do so before Yom Kippur. The Mishna states, “Yom Kippur atones for sins that are between man and God; however, Yom Kippur does not atone for sins which are interpersonal, until the offender has placated his friend” (Yoma 85b). Even if he were to sacrifice all the animals in the world, and pray and fast extensively, he would be not forgiven until he appeased his friend (Bava Kama 92a).
One who damaged his friend’s goods or his friend’s honor has sinned against both his friend and heaven. Therefore, not only must he placate his friend, he must also confess his transgression to God and resolve not to repeat it. So if a person injured his friend, stole something from him, or damaged his property, he should first pay his friend what he is owed and ask forgiveness for having hurt him. Only afterward should he confess his sin before God. Similarly, someone who embarrassed his friend or spoke of him in a belittling manner must first placate his friend and only then confess before God. If he confessed first, his confession is inadequate. He must confess again after he has made things right with his friend (Rabbeinu Yona, Sha’arei Teshuva 4:18).
Let us say that a person insulted someone or spoke ill of him in front of others, who might now treat him poorly as a result. The offender must make a point of speaking highly of this person in front of the same people. He must correct himself and explain why he was mistaken when he spoke badly of him (for example, he did not have the complete picture). This is to negate any damage he may have done. Similarly, it is proper for someone who insulted another publicly to ask for forgiveness publicly as well (MT, Laws of Repentance 2:5).
One must deeply believe in the possibility of teshuva and be certain that even just thinking about repentance rectifies much of what is wrong with him and with the world. Every thought of repentance increases the happiness and satisfaction in his soul…. If he has committed interpersonal sins but is not strong enough to ask forgiveness from his friend and correct them, he should not be discouraged from repenting altogether. After all, the sins relevant to his relationship with God, for which he has repented, are forgiven. The remaining sins that he has not yet corrected are only a small minority in comparison…. Nevertheless, he should continue to try very hard to avoid sinning within the interpersonal realm and make every effort to rectify what he can with wisdom and great courage… (Orot Ha-teshuva 7:6)
If one makes a serious resolution to avoid future sins in the interpersonal realm and does his best to rectify what was already done…spiritual light will continue to illuminate his soul. Eventually his spiritual courage will present him with many ways to complete the practical aspect of teshuva, so that spiritual light, in all its grandeur and goodness, will be able to dwell in his soul, which yearns for it. (Ibid. 10:6)
Even though one must do complete teshuva for any sin…even more so for interpersonal ones including theft and the like…nevertheless even if he was unable to do complete teshuva, even in the interpersonal realm…because something prevents him, he should not let it confound his expanded consciousness and the spiritual joy of understanding the supernal light and clinging to it. He should always think that he is benefiting the whole world by increasing divine light in his soul, which is a part of all the worlds and all the souls in general, and all the Jewish souls in particular. If so, he illuminates and benefits them, even those he hurt, and this is also a type of rectification of interpersonal harm, even though it is not enough… (Shemona Kevatzim 1:827)