Sahih Bukhari Volume 3, Book 46, Hadith Number 704.

Narated By Abu Huraira : The Prophet said, “Whoever frees his portion of a common slave should free the slave completely by paying the rest of his price from his money if he has enough money; otherwise the price of the slave is to be estimated and the slave is to be helped to work without hardship till he pays the rest of his price.”