What is the ruling on a merchant buying a man's goods from a factory—goods which their owner cannot clear except with the merchant's help—then the merchant sells them in the market for a dinar less than their original price?

This transaction is impermissible because it is a loan that draws a benefit, which is the buyer's advantage of purchasing the merchandise for less than the market price. This is forbidden usury (riba) by the consensus of scholars. The lawful solution is for the merchant to give the man a sum of money as a Mudarabah (profit-sharing) partnership for him to buy and sell the merchandise, and they would divide the profit according to their agreement.