Who is responsible for Zakat al-Fitr of a slave whose ownership is conditional upon sunset on the last day of Ramadan?

Primary text

If the sun sets while a slave is under an option period (khiyar) following a sale, or if a slave is gifted and accepted but not yet received, or if purchased but not yet received, the Zakat al-Fitr is incumbent upon the buyer or the donee because ownership belongs to them, and Zakat al-Fitr is due from the owner.