‘Amr b. Shu'aib, on his father’s authority, told that his grandfather reported God's Messenger as saying, “If anyone has agreed to let his slave buy his freedom for a hundred uqiyas and he pays them all but ten (or he said, ten dinars), and he is then unable to complete the amount, he remains a slave.”
Tirmidhi, Abu Dawud and Ibn Majah transmitted it.