Is it valid to specify the absolute best quality ('Ajwad) for the object of a forward sale?
Chapter on Guaranteed Salaf (Advance Payment) for a Specified Term
Al-Mughni
Book of Sales
Primary text
Specifying the absolute best quality ('Ajwad) invalidates the contract because the seller cannot guarantee the delivery of that which is exceptionally rare. If they do manage to provide it, it would be an anomaly.
Supporting text
If the absolute worst quality ('Arda') is specified, there is a differing opinion. One view holds it is invalid for the same reason of extreme rarity, while another view suggests it is valid because the seller can always deliver something better than what was stipulated, meaning they are never incapable of fulfilling the required delivery.