Why Abimelech Had to Ask Abraham Himself for Forgiveness

Our rabbis taught: all those amounts that the sages stated concern the payment for a person's humiliation, but for his pain, even if he brought all the rams of Nebaioth, he is not forgiven until he asks pardon of the injured party himself, as it says, "And now, restore the man's wife, for he is a prophet" (Genesis 20:7). Is it only a prophet's wife that is to be restored, and the wife of one who is not a prophet need not be restored? Rather, this is what He said to him: "And now, restore the man's wife in any case"; and as for what you said, "Will You slay even a righteous nation? Did he not himself say to me, She is my sister?" (Genesis 20:4-5) He is a prophet, and he has already learned from you. A guest who comes to a town, do you ask him about matters of food and drink, or do you ask him about matters of his wife, whether "She is your wife" or "She is your sister"? From here we learn concerning a descendant of Noah that he is put to death for what he ought to have learned and did not learn.