Is a restaurant contract, in which payment is made after eating or receiving the food, considered valid? And does it fall under the category of a salam contract, knowing that a salam contract requires payment of the price at the time of the contract?

The salam sale differs from a conventional sale. A salam sale involves a described item for which the responsibility lies with the seller (fi al-dhimmah), and it requires immediate payment of the price and a specified future date for the delivery of the sold item. This differs from other types of sales, which involve existing physical objects or non-existent objects described by their characteristics, and do not require immediate payment or a specified date. Eating in restaurants and purchasing food from them is not a salam sale; rather, it falls under the general category of sales.