Why is a portion of the slave manumitted if the entire *kitabah* debt is not paid when the manumission is enacted during the master's terminal illness?

Primary text

Manumission is enacted in part because the manumission occurred through the master's direct act of freeing the slave during his terminal illness, causing the act to be effective against one-third of the master's wealth, leaving the remainder for the heirs. This differs from manumission based solely on the *kitabah* contract, which requires full payment of the debt because, if any portion remains unpaid, the exchange is not fully realized, and freedom is not established in the consideration (*iwad*).