permissible to exchange one price for the other, a sale at either price, without specifying which, is permissible even when binding on both parties or one of them. This is because it is known that the sale becomes binding at the higher price if the seller has the choice, or at the lower price if the buyer has the choice: there is no doubt which price that party will choose, since there is no reason to prefer the other.
As for the second possibility—where exchanging one of the two prices or items for the other is permissible—it comprises four categories: (first) where the alternative prices or items belong to two different categories, such that one may be given as advance payment for the other; (second) where they belong to the same category but differ markedly in their characteristics; (third) where they belong to the same category and have the same characteristics, but differ in quality; and (fourth) where they belong to the same category, have the same characteristics, and are equal in quality.
Section
If they belong to two different categories, such that one may be given as advance payment for the other, the sale is permissible only according to the position of ‘Abd al-‘Azīz ibn Abī Salama. The same applies if they belong to the same category but differ so markedly in their characteristics that one may be given as advance payment for the other. If, however, they belong to the same category and differ markedly in their characteristics, but not to an extent that permits one to be given as advance payment for the other, the sale is permissible according to Ibn al-Mawwāz and also according to the position of ‘Abd al-‘Azīz ibn Abī Salama. If they belong to the same category but differ in quality, the sale is permissible according to what is stated in al-Mudawwana, the position of Ibn al-Mawwāz, and the view of ‘Abd al-‘Azīz ibn Abī Salama, but impermissible according to Ibn Habīb.
If they belong to the same category and have the same characteristics, the sale is permissible according to all the scholars of our school, contrary to al-Shāfi‘ī and Abū Hanīfa—may Allah have mercy on them both—who maintain that the parties may not part without settling on a known price. The evidence supporting our position is that the price is known, and allowing a choice between the two garments has no effect on the price. Rather, it concerns identifying the item sold, which does not invalidate the contract, just as when someone purchases a qafīz-measure of wheat from a heap containing several such measures.
Section
The sale becomes binding on both parties once it has been concluded verbally, even if they have not physically parted, unless