Nasai

Benefits and Issues:
➊
Sometimes shopkeepers, when they do not have certain items desired by their customers, immediately have them brought from the market and provide them. The intention behind this is that the customer remains attached only to them. This situation is not permissible. One should only sell the goods that are present. It is not allowed to sell what is not available, except if the customer himself requests the shopkeeper to have the item brought and provided.

➋
If an animal has run away, selling it is not permissible. Or if some property is disputed between the two parties, then selling it before a decision and possession is not allowed.

➌
If something has been purchased but not yet received, and it has not come into one’s possession, then selling it is impermissible. It should be noted that, according to the well-known commercial practice, the transaction of forward sale (bay‘ al-salam) is permissible, the details of which have already been mentioned.

Benefits and Issues:
➊
The clarification of the prohibited form is as follows: The seller does not possess an item, yet he determines its price and receives it, and says:
“When that item comes to me, I will give it to you.”
It is not known whether that item will come or not, or if it comes, whether the buyer will like it or not, or the item may turn out to be of lesser quality than the price paid.
There is a risk of disagreement and dispute arising between the two, therefore this form is prohibited.

➋
The sale of an unspecified item is also included in this, for example:
Before casting a net into the river, it is said that whatever fish come into the net, I sell them to you for such-and-such amount, while it is not known whether few or many fish will come into the net, whether they will be small or large, or whether any will come at all. Therefore, when the fish are brought out, then their sale can take place.
The first form falls under the sale of uncertainty (bay‘ al-gharar),
which is prohibited. (See Hadith: 2194, 2195)

➌
If the type, quantity, and characteristics of the item are determined, and the time of payment is also fixed, then it is permissible to pay the price in advance and receive the item at the appointed time later.
This is called forward sale (bay‘ al-salam or salaf). (See Hadith: 2280, 2282)

Urdu footnote:
"Do not make the deal," because it is possible that you may not find that item in the market, or you may find it at a price higher than the one you agreed upon, and then a dispute may arise. However, if the deal is not for a specific item but rather for a general item that is available in the market, and the buyer knows that the seller does not have it at present and will bring it from the market, then, insha Allah, there is no harm in making such a deal, as will be clarified in the previous hadith in the discussion regarding salam or salaf sale.