What are the rules regarding the owner accepting less of a better item or more of a lesser item when the usurped item is mixed?
General Chapter
Al-Mughni
Book of Usurpation
Primary text
If the usurped item is mixed with something inferior, and the owner agrees to accept the equivalent of his share from that inferior item, the usurper is obligated to provide it, as he was able to return part of the usurped good and the equivalent of the rest without harm. If the owner agrees to take less of the superior item, or the usurper voluntarily gives more of the superior item than is due, it is permissible because the excess is gratuitous and has no compensation (*iwad*).
Supporting text
If the owner agrees to accept more of the inferior item or less of the superior item, it is impermissible because it constitutes usury (*riba*), as the excess in quantity is taken as compensation for quality. If the parties agree on unequal exchange when mixing with a non-identical item, it is permissible because the exchange is for something other than its genus, thus rendering the excess permissible between them.