Is the expiation for Zihar permissible before the act of Zihar itself?

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The expiation for Zihar (a ruling concerning forbidden relations) is not permissible before the act of Zihar occurs. This is because a consequence cannot precede its cause. If a master states to his slave, 'You are free now in exchange for my future Zihar if I perform Zihar,' the manumission is ineffective regarding the Zihar expiation if the Zihar is subsequently performed. This is analogous to presenting the expiation for an oath before the oath is taken, or the expiation for accidental killing before the injury occurs.