Musannaf Ibn Abi Shayba 13:86

Hushaym narrated to us, from Yunus, from al-Hasan and Ibn Sirin, that they both disliked for a man to give his slave a dirham on the condition that he increases the yield for him. Ibn Sirin said: "He may give him a ransom, or a beast, or other such gifts, and increase upon it whatever he wishes."