Sahih Bukhari Volume 003, Book 044, Hadith Number 672.

Narated By Abu Huraira : The Prophet said, "Whoever manumits his share of a jointly possessed slave, it is imperative for him to get that slave manumitted completely by paying the remaining price, and if he does not have sufficient money to manumit him, then the price of the slave should be estimated justly, and he is to be allowed to work and earn the amount that will manumit him (without overburdening him)".