Musnad Ahmed: Hadith 1983

The Messenger of Allah (ﷺ) said with regard to the mukatab [a slave who has drawn up a contract of manumission with his master and is buying his freedom in instalments]: “A portion of the diyah of a free man should be paid, commensurate with how much he had paid towards his manumission, and a portion of the diyah of a slave should be paid, commensurate with the extent to which he was still a slave.”