What happens if a sane person is attacked by an insane person, and the sane person cuts the hand of the insane person that is not subject to Qisas?
Chapter on Retaliation (Qawad)
Al-Mughni
Book of Wounds
Primary text
If a sane person cuts the hand of an insane person that is not subject to Qisas (i.e., the non-retributable hand), the sane person's 'Aqilah (paternal relatives) are liable for its Diyya, and the sane person retains the right to Qisas for the other hand.
Supporting text
There are two views regarding cutting the retributable hand. One view is that the right to Qisas is waived because his right was specific, and taking it forcibly is like destroying a trust deposit. The second view is that the right is not waived; the sane person is entitled to the Diyya for his hand, and the Diyya for the perpetrator's hand is due from the perpetrator's 'Aqilah because the insane person cannot validly execute retribution. This differs from a destroyed trust deposit because the hand has compensation even if destroyed without negligence, unlike the trust deposit.