What constitutes valid dower (Sadaq)?

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The dower (Sadaq) must be property (mal). This is evidenced by the statement: "that you seek by your wealth" (Quran 4:24). The property must be such that if the marriage is dissolved before consummation, the remaining half retains a lawful value for the wife to benefit from. Consequently, items that cannot be a price in a sale, such as that which is forbidden (haram), non-existent (ma'dum), unknown (majhul), that which has no benefit, that which ownership cannot be fully established over (like sold measured or weighed goods before receipt), that which cannot be delivered (like a bird in the air or fish in water), and items not usually valued (like a single grain of wheat or a nutshell), cannot constitute dower.