Should the obligatory Mahr be one-fifth of the specified Mahr or one-fifth of the Mahr of the equivalent (mahr al-mithl)?
General Chapter
Al-Mughni
Book of Marriage
Primary text
The obligatory amount is one-fifth of the specified Mahr (khums al-musamma). This is derived from the narration of Uthman, whose ruling indicates that he mandated one-fifth of the specified amount, evidenced by the statement, "And her Mahr was five camels." Furthermore, if Mahr al-mithl were considered, it would necessitate awarding the entirety of that value, similar to how the full value is required for all damaged property, and it would necessitate payment in currency (athman) rather than camels.
Supporting text
It is possible that one-fifth of the Mahr of the equivalent (khums mahr al-mithl) is obligatory, as the Mahr serves as compensation for an injury (janayah), thus requiring reference to the value of the locus (mahall), just as in other penalties for bodily harm, and the value of the locus is the Mahr of the equivalent.