What is the ruling on a seller stipulating that he receives the down payment in the event that the buyer reneges on the purchase?

It is permissible to sell with an earnest payment—where the buyer pays the seller an amount that is part of the price upon completion of the sale, and which becomes the seller’s if the purchase is not completed—unless there is an authentic prohibition against it. Certain sales are excluded, such as salam (forward) sales and currency exchange, due to the requirement of complete immediate possession or full payment of the price. It is valid provided that the waiting period is specified and both parties agree.