Yom Kippur 5778

The Mishnah in Yoma (8:9) notes that Yom Kippur only atones for transgressions between Man and G-d, but does not atone for transgressions between and Man and his friend until he appeases his friend. Rabbi Elazar ben Azariah brings a verse from the Yom Kippur service to show this: "You shall be purified from yours sins before G-d" (Leviticus 16:30). Before G-d, but not friends.
Rabbi Joseph B. Soloveichik asks on the need to bring a verse for this lesson of appeasement. The Mishnah in Baba Kamma (8:7) teaches that when one injures his fellow Jew, the former must pay numerous fees such as for damages, embarrassment, and doctor bills. Even when payment is complete, the person is not forgiven until he seeks mechila (pardoning) from his friend. We learn this from Abraham and Avimelech. G-d told Avimelech to ask Abraham for forgiveness after taking Sarah away from him; this would lead to G-d healing Avimelech. Based on this Mishnah, we know that there is a principle to ask for forgiveness from your friend when you do something wrong. Why does Rabbi Elazar need to bring a second verse?
The Rav answers that Rabbi Elazar recognizes that there is a general rule of asking forgiveness which applies throughout the year. Here, however, he is only addressing the atonement of Yom Kippur. With Yom Kippur, it's not just about asking for forgiveness. It's about actually appeasing your friend and reverting back to the friendship that existed before any wrongdoing. You are truly making up with your friend and your relationship with him.
This is crucial to accomplish because the atonement of Yom Kippur is communal. We want to include everyone within the community without any trace of separation or division among us.
The Rav goes onto add that some rabbis from the mussar movement say that Yom Kippur won't atone for transgressions between Man and G-d if one doesn't appease his friend to resolve his transgressions between him and his friend first. Why is this?
The same Mishnah above ends with Rabbi Akiva calling G-d the mikveh of Israel. G-d is Who purifies us. However, a mikveh only works in totality. You can't purify just one part of your body; it must be your whole body. So too here, where you must be completely free of transgressions from every aspect and not just in one category.

Ki Teitzei 5777

In 25:13-16 the Torah tells us not to cheat in business. We shouldn't own different weights and measures to increase our own wealth as it is a form of thievery. The Gemara in Baba Basra (88b) discusses the severity of cheating in business to the point that it is harsher than the forbidden relationships mentioned in Leviticus 18. In both instances, the Torah calls "these actions" (in Hebrew "eleh") abominations. However, in Leviticus 18:27 "eleh" is spelled with one less letter; in our parsha, it is spelled out fully. The Gemara notes that a fuller spelling indicates a stronger language and therefore cheating business is harsher.
Why is this so? The Gemara concludes that one can do teshuvah if one sinned with forbidden relationships, but cannot when cheating in business. Rashi notes that when a person steals from the community, it is an impossible situation. When one steals, one is required to return the actual object to its rightful owner. However, it is very difficult to return the exact money to its rightful owner, let alone a whole community. While it possible for the thief to use those funds towards communal needs, it is not deemed a full teshuvah.