Ruling on forfeiting the right to inheritance in a misyaar marriage
Question

                Is it permissible in a misyaar marriage for the wife to forfeit the right to inheritance at the time of writing the marriage contract, if her husband dies when she is still married to him?.
Praise be to Allah.
      If the misyaar marriage fulfils the conditions and 
      necessary parts of marriage and is free of any impediments, then it is a 
      valid marriage in which all the consequences of marriage take effect, 
      including inheritance. So each spouse inherits from the other. If one of 
      the spouses stipulates that the other should forfeit his or her 
      inheritance in the event of death, this stipulation is not valid because 
      the legacy cannot be taken possession of until after the death of the one 
      from whom it is inherited. Forfeiting it before that is forfeiting 
      something that one does not possess. But if this forfeiting comes after 
      becoming entitled to the inheritance, which is after the death of the 
      person concerned, and is done willingly and freely by the one who is 
      forfeiting it, then it is permissible. 

    Shaykh Bakri al-Sadafi (may Allah have mercy on him), the 
    former Mufti of Egypt, said: If they get married on condition that one will 
    not inherit from the other if he or she dies, this condition carries no 
    weight and either inherits from the other if he or she dies. 

    End quote from Fataawa al-Azhar. 

    And Allah knows best.