"Sale" within the context of Sharia is understood as the act of exchanging one item of value for another, contingent upon the mutual consent of the parties involved.

Rule 1: The subject of sale must exist at the time of sale.

The item for sale needs to be present at the time the sale occurs. Consequently, an item that has not yet materialized cannot be subject to sale. Even if there is mutual agreement between the parties, the sale of an item that does not exist is null and void according to Sharia. For example, the transaction involving the sale of the offspring of an unborn animal is considered invalid. Comment: Exceptions to this rule include Salam and Istisna contracts.

Rule 2: The subject of sale must be in the seller's ownership at the time of sale. 

Ownership by the seller at the time of sale is a prerequisite for a valid sale. If a sale is attempted before the seller has secured ownership of the item, such a sale is deemed invalid. For example, when seller A attempts to sell a car to buyer B that A intends to purchase from seller C for resale to B, but A does not yet own the car at the time of the sale, this transaction is considered invalid.

Rule 3: The subject of sale must be in the physical or constructive possession of the seller at the time of sale. 

For a sale to be valid, the seller must either physically possess the item or have constructive possession of it at the time of sale. Constructive possession implies that while the seller or his agent may not have physically taken delivery of the item, it is under his or his agent's control, and the buyer has assumed all rights and responsibilities for it, including the risks associated with its damage or loss. For example, (case 1) buyer A purchases a car from seller C but neither he nor his agent has yet taken delivery of it, A is not in a position to resell the car to buyer B. If he sells it before taking its delivery from C, the sale is void. However, (case 2) if seller C has placed the car in a garage, granting A or his agent access and the right to retrieve it at will, all risks associated with the car are transferred to A, then A can resell the car to B, even without physical possession.

Comment 1:

The gist of the rules mentioned in paragraphs 1 to 3 is that a person cannot sell a commodity unless:

1. It has come into existence.
2. It is owned by the seller.
3. It is in the physical or constructive possession of the seller.
4. The rules mentioned in paragraphs 1 to 3 are relaxed for two specific types of sales, namely Salam and Istisna contracts.

Comment 2:

There is a big difference between an actual sale and a mere promise to sell. The actual sale cannot be effected unless the above three conditions are fulfilled. However, one can promise to sell something that he does not yet own or possess. This promise initially creates only a moral obligation on the promisor to fulfil his promise, which is normally not justifiable. Nevertheless, in certain situations,especially, where such a promise has burdened the promisor with some liability, it can be enforceable through the courts of law. In such cases the court may force the promisor to fulfil his promise, i.e. to effect the sale, and if he fails to do so, the court may order him to pay the promisee the actual damages incurred due to the promisor's default. But the actual sale will have to be effected after the commodity comes into the possession of the seller. This will require separate offer and acceptance, and unless the sale is effected in this manner, the legal consequences of the sale shall not follow. The reason is that unenforceable promises in commercial dealings could endanger business activities. The Islamic Fiqh Academy in Jeddah mandates that promises in commercial dealings be binding on the promisor if they meet these criteria:

• The promise is unilateral.
• The promise incurs liabilities for the promisor.
• If the promise involves a purchase, the actual sale must occur at the specified time through an exchange of offer and acceptance. A mere promise is not considered a finalized sale.
• If the promisor reneges, the court may compel them to complete the purchase or compensate the seller for actual damages, excluding opportunity costs.

Rule 4: The sale must be immediate and absolute. 

A sale must immediately and unconditionally transfer ownership rights from seller to buyer. Any sale that is scheduled for a future date or is contingent on a future event is deemed void. For example, if seller A tells buyer B that he will sell his car in a month or when a new batch of cars of a specific brand arrives at the dealership, such a sale is considered void. The parties must renegotiate and execute a valid transaction when the specified future event or condition occurs.

Rule 5: The subject of sale must have value according to trade customs. 

The item being sold must possess value as recognized by customary trade practices. Items deemed valueless or without demand cannot be sold or bought, as this would contravene the principles of legitimate trade transactions. Creating an artificial demand to mask financing transactions as trade activities is strictly prohibited.

Rule 6: Trading is permissible only with halal (permissible) items. 

The sale must involve items that are permissible under the Sharia. Selling items that are inherently haram, such as pork or wine, contradicts the norms and rules of the Sharia and is therefore invalid.

Rule 7: The subject of sale must be specific, known, and defined for the buyer. 

The subject of sale must be clearly identifiable. For example, seller A, a car dealer, cannot sell a car to buyer B without explicitly identifying and defining the specific vehicle. Ambiguity in the identification of the sale item renders the sale void. For example, seller A, a car dealer, cannot sell an unspecified car to buyer B without specifying a particular vehicle.

Rule 8: The delivery of the sold product to the buyer must be definite. 

The delivery of the sold item must not rely on random circumstances or chance. For example, if seller A's car is stolen, but A decides to sell the stolen car, and buyer B agrees to purchase it in the hope that either they or the police will catch the thief and B will gain ownership rights, such a transaction would not be considered valid.  

Rule 9: The price must be known and determined. 

The price of the item being sold must be clear and agreed upon by all parties involved in the transaction. For example, if seller A offers a car for $50,000 for immediate payment or $55,000 if the payment is delayed for a year, the parties must agree on one of these prices for the sale to be valid.

Rule 10: The sale must be unconditional. 

The sale contract should exclude conditions unrelated to the transaction. Only conditions that are part of the sale and recognized according to trade customs are valid. For example, (case 1) if seller A sells a car to buyer B with the stipulation that B must employ A's son, such a condition invalidates the sale because it is extraneous to the transaction and not in accordance with trade usage. However, (case 2) if buyer B purchases a car from dealer A with the condition that A provides complimentary maintenance services, such a condition is valid as it directly relates to the sale itself. For instance, if buyer B purchases a car from dealer A on the condition that seller A provides free oil, filter, and spark plug replacement every 40,000 km for two years, such a condition is considered valid because it is part of the transaction in accordance with trade usage.