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).
أَنَّ نَبِيَّ اللَّهِ ﷺ قَضَى فِي الْمُكَاتَبِ أَنْ يُودَى بِقَدْرِ مَا عَتَقَ مِنْهُ دِيَةَ الْحُرِّ