What is the distinction regarding the punishment for slander when the wife confirms the husband's adultery versus when she affirms her own?

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If a man calls his wife 'O adulteress,' and she replies, 'You are more adulterous than I am,' the ruling stated by Imam Abu Bakr is that the hadd against her is waived, similar to the case where she simply confirmed his accusation against herself. However, in this scenario, the punishment for slander against the husband (by the wife) becomes incumbent upon her, unlike the previous case, because she attributed adultery to him, whereas previously she attributed it only to herself.