Ata narrated from Jabir bin Abdullah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "There is no harm in selling the service of the mudabbar when (his owner) is in need."
This is an error on the part of Ibn Tarif.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21551