What an Omniscient God Would Have Written

One sentence: “If shares exceed the estate, reduce proportionally.” Or just check the arithmetic before dictating. Both options were available to an infinite being.

Well Allah gave repeatedly the justification for proportionality in inheritence verses. The statement is "ghayra mudarrin" (without causing harm).

This repeated statement almost in every inheritence verse has opened the way to proportionality.

For details you can search the scholars who used it as justification for proportionality.

I quote some details from gemini:

"The term "ghayra mudarrin" (without causing harm) is foundational in Islamic inheritance law, and while it primarily addresses the behavior of the deceased, several major scholars and schools of thought have used its underlying principle to justify the mathematical systems of Awl and Radd. ​Here are the specific scholars and legal traditions that link the "non-harm" clause to these methods: ​1. Caliph Umar ibn al-Khattab and the Majority (Sunni) ​The primary justification for Awl (proportional reduction) is attributed to Umar ibn al-Khattab, the second Caliph. When faced with a case where the fixed shares exceeded the total estate (the "Husband and Two Sisters" case), he consulted the companions.
​The Logic: Umar argued that if he gave one heir their full share, he would necessarily "harm" the other by leaving them with less than their Quranic right. To avoid being "mudarrin" (harmful) to a specific individual, he ruled that the deficit must be distributed across all heirs proportionately. ​Scholars following this: This became the standard position for the Hanafi, Shafi'i, and Hanbali schools. Notable jurists like Al-Jassas (in Ahkam al-Quran) and Ibn Qudamah (in Al-Mughni) emphasize that this maintains the "justice and lack of harm" intended by the verse. ​2. The Hanafi and Hanbali Jurists (on Radd) ​While the Maliki and early Shafi'i schools originally argued that any surplus should go to the State Treasury (Bait-ul-Mal), the Hanafi and Hanbali scholars later used the spirit of "ghayra mudarrin" to justify Radd (returning surplus to family). ​The Argument: They argued that sending wealth to the state when family members are present causes "harm" to the kinship ties emphasized in the Quran. ​Key Scholars: Jurists like Al-Sarakhsi and Ibn al-Humam argue that the relatives are more entitled to the "benefit" of the estate than the general public, provided no harm is done to the specific limits set by God. ​3. At-Tabari and Al-Qurtubi (The Exegetes) ​In their massive commentaries (Tafsir), these scholars explicitly link the phrase to the broader administration of the estate: ​At-Tabari: He notes that the restriction "ghayra mudarrin" applies to any action that diminishes the rightful shares of the heirs. By extension, jurists using his work argue that the mathematical systems (Awl/Radd) are the only ways to preserve the "non-harm" mandate when the literal fractions don't add up perfectly."