How is the compensation (excess over Mahr) calculated in a Khul' when the wife dies, according to Shafi'i?

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Shafi'i asserts that the excess compensation beyond the Mahr al-Mithl is a gift (muhabah) that is counted from one-third of the wife's remaining estate after the Mahr al-Mithl is secured for her. For example, if the Mahr al-Mithl is twelve and the compensation is thirty, he receives eighteen: twelve for the Mahr and six as one-third of the remaining estate six.