Is the emancipation (freeing of a slave) valid if performed by someone legally incompetent to dispose of property?

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Emancipation performed by a minor (Sabi) or an insane person (Majnun) is invalid. This is the position of the general body of scholars, including Al-Hasan, Al-Sha'bi, Al-Zuhri, Malik, Al-Shafi'i, and the Ahl al-Ra'y (the companions of reason/opinion). The evidence for this is the saying of the Prophet (peace be upon him): "The pen is lifted from three: the child until he reaches puberty, the insane person until he recovers sanity, and the sleeper until he wakes up." Furthermore, emancipation is a donation of wealth, and it is not valid from them, similar to a gift (Hiba).