سنن ابن ماجه

It was narrated from `Amr bin Shu'aib , from his father, from his grandfather that the Messenger of Allah (ﷺ) said: “Any slave who has made a contract to buy his freedom for one hundread Uqiyyah and pays it all except ten Uqiyyah; he is still a slave.” (One Uqiyyah is equal to 40 Dirham.)