It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the case of a mukatab was brought before the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "In proportion to what he has paid of his contract, he should pay the blood money of a free man, and in proportion to what he has not paid, the blood money of a slave."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11943