It was narrated from 'Aishah that:
a man bought a slave and put him to work, then he found some defect in him, so he returned him. He (the seller) said: "O Messenger of Allah he put my slave to work." The Messenger of Allah (ﷺ) said: "A slave's earnings belong to his guarantor." (IT)
English reference (IT): Vol. 3, Book 12, Hadith 2243 (IT)
Arabic reference (IT): Book 12, Hadith 2328 (IT)