Sunan an Nasai
4808
Sahih+2

It was narrated that Ibn 'Abbas said:"The Messenger of Allah ruled that the Diyah for a Mukatab who is killed should be (equivalent) to the Diyah for a free mand, proportionate to the amount be had paid off (toward buying his freedom)

SahihAbu Ghuddah
SahihAl-Albani