وقال محمد: يجب العشر إذا بلغ الخارج خمسة أمثال من أعلى ما يقدر به نوعه
Translation: Muhammad says ushr is due when the yield reaches five times the higher customary measure used for its type.
Muhammad gives a different threshold: for a product not measured in awsuq, use five units of its own larger customary measure.
Thus the product’s measurement convention, rather than a valuation against the cheapest qualifying grain, determines the threshold.