What is the ruling on selling building materials that the merchant does not possess at the time of sale, then he buys them and delivers them to the customer, or selling them at the current day's price and delaying delivery while keeping the agreed-upon price fixed?

This scenario is Islamically permissible if it falls under the category of a Salam (forward) sale. It has specific conditions that must all be met in the contract for it to be valid, and these are seven: that its description can be precisely defined, that it is described in a way that clearly distinguishes its price, that its quantity is mentioned by measure, weight, or linear dimension, that a known delivery date is stipulated, that it is generally available in its locality, that the price is paid in full at the contract session, and that it is delivered based on a liability (Dhimmah).

The basis for the permissibility of Salam is the Almighty's saying: {O you who have believed, when you contract a debt for a specified term, write it down}, and the Prophet's (peace be upon him) saying: "Whoever makes a forward payment (salaf) for something, let him make it for a known measure, a known weight, and to a known term."

If these conditions are met, then a change in the market price of the commodity does not affect the validity of the sale.