What is the ruling if working the mine is permitted on the condition of paying the owner a known sum?

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If the owner states, 'Work in it, and you may have what is extracted, provided you give me one thousand or a known amount of copper,' the agreement is invalid. This is deemed a sale of the unknown (*bay' li-majhul*). It is also invalid as a form of joint venture (*mu'amalah*) like Mudarabah for the aforementioned reasons, and because Mudarabah requires a share of the growth, not a specified amount of currency.