What is the ruling concerning a slave upon whom the obligation of 'Si'ayah' (a determined payment for freedom) has been imposed, when the first master frees him partially?

Primary text

If a slave upon whom 'Si'ayah' is incumbent is partially manumitted by the first master, the validity of the second master's manumission depends on when the slave's freedom is established. Those who rule that freedom is secured upon the first act of manumission (even partial) invalidate the second manumission, because the second act is deemed an emancipation contingent upon the first emancipation. Conversely, those who do not consider the slave free until the completion of the 'Si'ayah' validate the second manumission, as the second master is freeing a portion of property that was rightfully his at the time of the second act.