What is the ruling if a deceased person's slave claims his master wrote him a Mukatabah (contract of manumission) and the heirs confirm it?

Primary text

If the heirs confirm the Mukatabah, it is established, as the right belongs to them. The slave remains in the Mukatabah, and upon fulfilling the payment to the heirs, he is emancipated. If the heirs refuse to release him, he may be returned to slavery if he becomes incapable (of fulfilling the contract), but if the heirs do not declare him incapable and remain patient with him, he cannot annul the contract himself.