Is it permissible to enter a Salam contract for a commodity that is to be delivered in unknown, distinct installments over time?
Chapter on Guaranteed Salaf (Advance Payment) for a Specified Term
Al-Mughni
Book of Sales
Primary text
It is permissible for a person to pay currency to another for an edible commodity, and take from that commodity daily. This is valid if structured on the meaning of a Salam contract. This is exemplified by a butcher receiving a Dinar to deliver a specified measure of meat daily. Malik holds this view. The underlying principle is that if the subject matter is deliverable in known, specified portions, it is permissible.
Supporting text
Al-Shafi'i has two opinions if a single commodity is contracted for with two different delivery periods (ajalin): one view holds it is invalid because the portion corresponding to the further delayed item would have less value, which is unknown. Our position is that every sale valid for one period is valid for two or more periods, analogous to spot sales. If part is received and the rest becomes impossible, the price is returned proportionally, and the received portion is not favored over the undelivered portion because the commodity consists of identical parts, so the price must be divided equally among them, as if the delivery periods were the same.