Is there a minimum threshold (Nisab) required for the Khums to be obligatory on Rikaz?
Chapter on Zakat on Gold and Silver
Al-Mughni
Book of Zakat
Primary text
The Khums is obligatory on both small and large amounts of Rikaz. This is the position of our Imam (Shafi'i), Malik, Ishaq, the companions of Ray, and Al-Shafi'i in his old opinion. The justification rests on the generality of the Hadith, and because it is a taxed property, no minimum threshold should be considered, just like spoils of war. Additionally, since it is property belonging to a disbeliever found in Islam, it resembles spoils of war. Minerals and crops require labor and effort, so a threshold is set for mitigation, unlike Rikaz.
Supporting text
The new opinion of Al-Shafi'i holds that a Nisab must be considered because it is a right upon wealth extracted from the earth, thus paralleling minerals and crops where a threshold is considered. Furthermore, in minerals and crops, the due is a form of equalization (muwasah), requiring a threshold to reach a level where such sharing is possible, which is not the case for Rikaz.