What is the ruling on making an exception in a conditional Zihar or Haram pronouncement?

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If a person states a conditional Zihar, such as, 'If I enter the house, then you are to me like my mother's back, if God wills' (In dakhalta ad-dara, fa-anti 'alayya ka-dhahri ummi, in sha'a Allah), or a conditional prohibition, 'You are forbidden to me, if God wills,' nothing is binding upon him, as the exception applies to the expiable oath embedded within the condition.