Ata' bin Abi Rabah (may Allah have mercy on him) says that Sayyiduna Ibn Abbas (may Allah be pleased with them both) said regarding a man who made a contract of manumission (mukatabah) with his slave in exchange for three young slaves: There is no harm in that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21631