Nafi' (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he used to pay sadaqat al-fitr on behalf of every slave of his who was on his land or elsewhere, and on behalf of every person whom he fed, whether young or old, and also on behalf of his wife's slaves; but he had mukatab slaves in Madinah, and he did not give on their behalf.
Sufyan al-Thawri (may Allah have mercy on him) says that Sayyiduna Ibn Umar (may Allah be pleased with them both) had two mukatab slaves, but he did not give sadaqat al-fitr on their behalf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7686