Imam al-Shafi'i (may Allah have mercy on him) said: "It is narrated that whoever makes a contract of manumission (mukatabah) with his slave for one hundred uqiyahs, and he (the slave) pays all of it except ten uqiyahs, he is (still) a slave."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / Q21636