Is a bequest freeing a female slave conditional upon her never marrying valid?

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If a master bequeaths the freeing of his female slave on the condition that she never marries, and then he dies, her manumission takes effect if she states she will not marry. If she marries later, her manumission is not invalidated. This is the position held by Al-Awza'i, Al-Layth, Abu Thawr, Ibn al-Mundhir, and the Ahl al-Ra'y (the proponents of juristic reasoning). This ruling is based on the principle that once manumission has taken effect, it cannot be revoked.