Does a seller's taking an increase in the price of merchandise over its original value, along with a request for an invoice with the new price to benefit from this increase, constitute usury (riba)? And what is the ruling on the one who does so?

If the owner of the goods agrees with a man that he will sell him the goods for a specific price, and whatever exceeds that is for him, then the contract is legally permissible according to the most preponderant opinion, and there is no usury in it, and the agent is entitled to this increase.