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It was narrated that Ibn 'Abbas said: "The Messenger of Allah (ﷺ) ruled that in the case of a Mukatab, the Diyah should be (equivalent) to the Diyah for a free man, proportionate to the amount he had paid off (towards buying his freedom)." (Dar'if)
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ إِسْمَاعِيلَ بْنِ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا يَعْلَى، عَنِ الْحَجَّاجِ الصَّوَّافِ، عَنْ يَحْيَى، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَضَى رَسُولُ اللَّهِ صلى الله عليه وسلم فِي الْمُكَاتَبِ يُودَى بِقَدْرِ مَا أَدَّى مِنْ مُكَاتَبَتِهِ دِيَةَ الْحُرِّ وَمَا بَقِيَ دِيَةَ الْعَبْدِ .
sunnah.com