[Requiring Specification of the Item Sold, Not the Price]

Answer: It is the nature of the price—gold, silver, and the like—that it is indeterminate, and that the seller has no interest in specifying a particular coin.
It is the nature of the item sold that it is, for the most part, not subject to ignorance, and that the buyer does have an interest in its identity.
Thus the requirement to specify the item rather than the price follows this; and by this stipulation the mutual consent required for trade by the text of the Qurʾān is realized. For the buyer is not pleased with the merchandise unless he knows it in its very identity; whereas the seller is content with payment in the currencies and has no interest in specifying particular pieces.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2