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.
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 ﷺ بَيْنَهُمْ