Is the aforementioned transaction - which takes place by agreeing to purchase a certain quantity of livestock feed from a wholesaler, who in turn buys it from the factory or warehouses and sends it to us by rented trucks - considered a sale by promise or a sale of what one does not possess? And is the contract deemed invalid in this manner, knowing that the obligation to [take] the goods only occurs upon their قبض (seizure/possession) and the receipt of their price?

It is permissible to request a commodity and promise to purchase it when the seller acquires ownership of it. It is not permissible for the seller to sell the commodity before purchasing and possessing it, due to the Prophet's (peace be upon him) prohibition of selling what you do not own. An exception to this is the sale of a described commodity that is a debt (salam contract), provided that the full price is paid at the contracting session. If the commodity is only purchased upon its arrival to the buyer, and it perishes before that while still with its seller, there is no harm. However, it is not permissible for the damaged or unwanted commodity to be returned directly to the manufacturer or warehouses; instead, it must be under the wholesaler's guarantee.