Is it permissible for the seller to keep the down payment if the buyer backs out of completing the purchase?

Scholars differ on the fate of the down payment (urbun) if the buyer withdraws from the deal. The majority of them hold that the sale with a down payment is invalid, and it must be returned to the buyer, based on what was narrated that the Prophet (peace be upon him) forbade it. Imam Ahmad, however, was of the view that it becomes permissible for the seller if they both agree to it, citing the action of Umar ibn al-Khattab (may Allah be pleased with him) as evidence. If the seller and buyer agree on a certain matter regarding the down payment, or if custom dictates it, then their agreement is acted upon. If they do not agree, and there is no custom, and the seller incurs damage due to the buyer's withdrawal, then the seller may estimate his efforts and what he missed out on, and the buyer is obliged to pay what corresponds to that loss. The seller may also compel the buyer to proceed with the sale and retain the down payment as a guarantee for his right until the transaction is completed and the seller receives the price, then return to the buyer the remainder of its price.