Rav Yosef Karo: Nirtza

“V’nirtza Avona,” “that her iniquity has been expiated”] means that the iniquity speaks in her favor, in the secret of iniquities becoming credits. Would you not ask the difficult question: How is this possible? You know that a sin committed by a person sits before the Holy One blessed be He and prosecutes the sinner, saying, ‘So and so transgressed the command concerning me on that day.’

But when a person repents, that same sin proclaims before the Holy One blessed be He, ‘that person who transgressed against me on that day, has repented.’ Thus the sin itself speaks in favor of that person. This is what is meant by “her iniquity has come to be in her favor,” i.e., her iniquity speaks in her favor before the Holy One blessed be He to say that the person repented of that sin.