I say, and with God's success: Zakat al-Fitr is not obligatory on the slave; rather, it is given by his master. This is because the obligation is the obligation of performance, and there is no way to impose performance on the slave; since the slave is not required to perform it at the moment nor after being freed. To impose an action that cannot be performed at all is impossible. Refer to: Al-Badai 2: 70, and Al-Wiqaya p. 229, and God knows best.