the Prophet of Allah ruled that the Diyah for a Mukatab should be (equivalent) to the Diyah for a free man, proportionate to the amount he had paid off (toward buying his freedom). (Using translation from Nasāʾī 4809)
أَنَّ رَسُولَ اللهِ ﷺ «قَضَى فِي الْمُكَاتَبِ أَنْ يُؤَدِّيَ بِقَدْرِ مَا عَتَقَ مِنْهُ دِيَةَ الْحُرِّ»