What is the ruling on a seller demanding an increase over the price paid in advance by the buyer, after a delay in delivery and a change in prices, and what is the ruling on selling construction materials where the price is paid in advance and delivery takes place after months or years?

If the sale contract has been concluded validly, the buyer is not obligated to pay an additional amount, and the seller must deliver the agreed-upon item. The seller bears his loss. However, if the seller requests an increase from the buyer and the buyer willingly accepts, there is no harm.

As for salam selling (forward buying), which is the purchase of a described commodity that is a liability (in trust), it is permissible if its conditions are met, such as specifying a known delivery date.