Question: How do we differentiate between countable (ma'dood), weighable (mawzoon), and measurable (makeel) items in the relevant Sharia rulings? In other words, what is the criterion for this?
Answer: Countable, weighable, and measurable items are determined according to the reality existing in the sale of commodities.
That is, if you want to know the permissibility of Salam (advance payment) in a certain commodity, look at how this commodity is sold in the market. Is it sold by number—meaning, is it called out in the market as "each piece for such-and-such" or "each one for such-and-such"? Is it sold by weight, where it is called out as "each kilo for such-and-such"? Or is it called out as "each meter for such-and-such"? Or "each sa’ for such-and-such"?...
If that is the case, the description of "countable," "weighable," or "measurable" applies to it, whether it is a description for one unit or more. That is, a commodity can be described as weighable and countable...
Examples: Wheat and barley... by weight and measure. Bananas and oranges... by weight and by number (in some countries, they are sold by number). Cloth... by measurement (the cubit and the meter). And so on. Whatever is regulated by its description upon receipt (qabd)—whether by weight, count, or measure, some or all of them—Salam is permissible in it. Regulating its description upon receipt through these matters is important to negate jahala (uncertainty). When it is said to me, "You have 100 kilograms of wheat" and nothing more, it is not enough to regulate the description; rather, the type of wheat must be specified so that the weight truly negates jahala. The same applies to measurable and countable items.
But are animals sold by number, such that one calls out saying, "Each camel is for a thousand"? Or are they sold by the buyer seeing and looking at the animal, saying, "This camel is not worth a thousand," and then choosing a second camel? Is every camel like the other such that the sale can be by number?
Furthermore, are houses sold by number, weight, or measure? Meaning, does a man who has ten houses call out, "A house for a thousand," or is the purchase of any house done by seeing it, as it differs from the other...
Therefore, it is said that animals and houses are not sold by measure, weight, or count. You might say that some people sell their animals by weight, but this is not the case for every animal. He might sell a specific sheep by weight, but he does not sell every sheep that way, nor does he sell every animal that way—he does not sell a cow by weight... As long as the sale of an animal is not always regulated by weight, Salam does not enter into it, as it cannot be said to me, "You have a hundred kilos of animal" (referring, of course, to it while it is alive).
Similarly, you might say that some people sell houses by the meter, but this does not apply to every house. Rather, the meter in this house is for ten, and in that house for twenty, and so on. Thus, regulating its description is not by measurement; it cannot be said to me, "You have a hundred meters of house..." And so on.
In any case, the general rule is that if a commodity's descriptions are regulated upon receipt by weight, number, or measure, Salam is permissible in it; otherwise, it is not. It can be said that most things are weighable, measurable, or countable, but it cannot be said that all things are.
The subject of stone (for construction) is as follows:
a- If the stones are regulated in description by measure or number upon receipt, then it is permissible to sell them via Salam. That is, the price is paid to the quarry owner for stones described by measure or number, and then after the specified period, the stones are received. I repeat: if their descriptions are regulated upon receipt by measure or number. Salam is permissible from someone who possesses the commodity or someone who does not. In this case, the prices are paid first, and the goods are received at the specified time.
b- Purchasing stones (not as Salam): In this case, the stones must be present with the seller due to the prohibition of selling what one does not own. The buyer sees them, and if he agrees, he buys them and pays the price—either all in cash, or some in cash and some as debt, and so on, according to the agreement.
c- If the matter is neither a sale nor Salam, but rather "hiring" (ujra). For example, if you want to build a stone wall, you contract a builder to build the wall for you as a Muqawala (contracting) at a certain rate per meter, including materials and labor... this is on the builder. This is a Muqawala contract. In this case, it is permissible to contract with the builder whether the stone is with him or not. He is committed to building the wall; this is a Muqawala contract and not a contract to purchase stone, neither through Salam nor on the spot.