Is it permissible for a guardian to perform a vowed fast (Nadhr) on behalf of the deceased?

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The guardian must perform the vowed fast (Nadhr) on behalf of the deceased. This view is held by Ibn Abbas, Al-Layth, Abu Ubayd, and Abu Thur. The evidence rests upon the authentic Hadiths mentioned previously and the Sunnah of the Messenger of Allah, which takes precedence over other opinions. The rationale distinguishing vows from other obligations is that substitution (niyabah) enters acts of worship according to their relative lightness; the vow is considered lighter because it was not originally mandated by the Lawgiver but self-imposed by the person making the vow.