Is a bequest (wasiyyah) concerning a freed slave's writings (mukātab) valid?

Primary text

The bequest concerning the writing (manumission contract) of a slave is valid because a bequest may concern something not yet fully established, similar to a bequest of unharvested fruit or a fetus. The legatee (Mawṣā lah) has the right to claim the due amount when it matures, or to forgive the debt. Upon the full settlement or forgiveness, the mukātab becomes free, and the master who granted the manumission retains the right of loyalty (al-walā') as he is the benefactor.