What invalidates an otherwise sound contract of manumission?

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Abu Bakr states that the contract of manumission is not invalidated by the master's death, insanity, or legal guardianship due to foolishness, because it is a contract of manumission, just like a sound one. This contract differs from pure manumission in that the slave's earnings before payment belong to the master, but any surplus after payment belongs to the slave. Furthermore, the children of the contract-maker follow the mother in one opinion, treating it like a sound manumission contract.