She promised her husband that she would never marry anyone after him
Question

                A woman gave her husband a promise that she would never marry anyone after him, until Allaah joined them by His mercy and grace. Does she have the right to marry anyone after him? Is this regarded as not fulfilling a promise which is one of the attributes of the hypocrites?.
Praise be to Allah.
    It is permissible for this woman to marry after her husband 
    dies, but she should offer expiation for her oath, because the Prophet  
    (peace and blessings of Allaah be upon him) said: “By Allaah, if Allaah 
    wills, I do not swear an oath then see something better than it but I will 
    offer expiation for my oath and do that which is better.” Narrated by 
    al-Bukhaari, 3133; Muslim, 1649. And he  (peace and blessings of Allaah 
    be upon him) said: “If you swear an oath then you see something that is 
    better than it, offer expiation for your oath and do that which is better.” 

    Narrated by al-Bukhaari, 2622; Muslim, 1652. 

    Marriage is better for this woman then her remaining 
    unmarried, because that serves many interests.