Is a bequest (Wasiyyah) made to one's Mukatab (contracted slave) valid?

Primary text

The bequest made to one's Mukatab, the Mukatab of an heir, or a Mukatab belonging to a non-relative is valid. This applies whether the bequest was of a common share or a specified asset. The basis for this ruling is that the heirs have no right over the Mukatab, nor do they possess his wealth.