From Sahih Muslim (Hadith 3774):
This hadith has been narrated on the authority of Sa'id b. Abu 'Aruba with the same chain of transmitters but with the addition:" If he (one of the joint owners emancipating the slave) has not (enough) money (to secure freedom for the other half) a fair price for the slave should be fixed, and he will be required to work to pay for his freedom, but must not be over-burdened
Reference: Muslim 3774