Abu Hurairah narrated:
“I heard the Messenger of Allah saying: ‘Whoever frees a believing slave, then Allah frees a limb from the Fire for each of his limbs, such that he frees his private parts in lieu of his private parts.” (Sahih)
This narration suggests that an honest and physically fit slave should be freed in expiation for breaking an oath. This will cause freedom of every limb of the person who manumits him from the Hellfire.
Jami At-tirmidhi volume 3, hadith number 1541