the Messenger of Allah ﷺ ruled that what a slave earns belongs to his guarantor.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَضَى أَنَّ خَرَاجَ الْعَبْدِ بِضَمَانِهِ  

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."  

أَنَّ رَجُلاً اشْتَرَى عَبْدًا فَاسْتَغَلَّهُ ثُمَّ وَجَدَ بِهِ عَيْبًا فَرَدَّهُ فَقَالَ يَا رَسُولَ اللَّهِ إِنَّهُ قَدِ اسْتَغَلَّ غُلاَمِي فَقَالَ رَسُولُ اللَّهِ ﷺ الْخَرَاجُ بِالضَّمَانِ