Narrated Abdullah ibn Abbas: When this verse was revealed: "If they do come to thee, either judge between them, or decline to interfere....If thou judge, judge in equity between them." Banu an-Nadir used to pay half blood-money if they killed any-one from Banu Qurayzah. When Banu Qurayzah killed anyone from Banu an-Nadir, they would pay full blood-money. So the Messenger of Allah (ﷺ) made it equal between them.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ النُّفَيْلِيُّ، حَدَّثَنَا مُحَمَّدُ بْنُ سَلَمَةَ، عَنْ مُحَمَّدِ بْنِ إِسْحَاقَ، عَنْ دَاوُدَ بْنِ الْحُصَيْنِ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ لَمَّا نَزَلَتْ هَذِهِ الآيَةُ { فَإِنْ جَاءُوكَ فَاحْكُمْ بَيْنَهُمْ أَوْ أَعْرِضْ عَنْهُمْ } { وَإِنْ حَكَمْتَ فَاحْكُمْ بَيْنَهُمْ بِالْقِسْطِ } الآيَةَ قَالَ كَانَ بَنُو النَّضِيرِ إِذَا قَتَلُوا مِنْ بَنِي قُرَيْظَةَ أَدَّوْا نِصْفَ الدِّيَةِ وَإِذَا قَتَلَ بَنُو قُرَيْظَةَ مِنْ بَنِي النَّضِيرِ أَدَّوْا إِلَيْهِمُ الدِّيَةَ كَامِلَةً فَسَوَّى رَسُولُ اللَّهِ صلى الله عليه وسلم بَيْنَهُمْ .
sunnah.com