Is the exception (Istithna') of animal fat in a sale valid?

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The exception of animal fat during a sale is invalid. Ahmad explicitly stated this ruling. Scholars agree with Abu Abdullah (Ahmad) that this is impermissible because the Prophet (peace be upon him) forbade the exception (*al-thunya*) unless it is known. Furthermore, since fat is indeterminate (unknown), its individual sale is invalid, and thus its exception in a larger sale is also invalid.