When this verse was revealed: "If they do come to you, either judge between them, or decline to interfere....If you 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.
Reference: Sunan Abi Dawud 3591 In-book : Book 25, Hadith 21 English translation : Book 24, Hadith 3584
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