For Mithl Ghayr Ma'qul, a transmitted legal text is necessary. One may not invent money as an equivalent for a life or bodily part merely through personal reasoning.
The closing legal example contrasts accidental killing with intentional killing. The stated Diyah rule for the accidental case cannot simply be transferred to compel payment in the intentional case instead of Qisas1.
The source also gives a distinct settlement case: the perpetrator voluntarily agrees to pay, and the victim's heirs do not want Qisas and accept settlement. Both sides of that consent must remain in the example.
This explains a limit on substitution. It is not authorization for personal punishment or a procedure for deciding a real case.
1. Qisas: the legally regulated retribution mentioned in this homicide example, not private retaliation.
- A Substitute Cannot Be Invented