Is it permissible for a wife in a Misyar marriage to waive her inheritance when the contract is written, in the event of her husband's death while she is still married to him?

If the conditions and pillars of a Misyar marriage are met and there are no impediments, then it is a valid marriage that entails the effects of marriage, including inheritance. The condition of waiving inheritance before the death of the inheritee is not valid because it is a waiver of something not yet owned. However, if the waiver occurs after the inheritance is due and the renunciant is willing, then it is permissible.