Is the profit resulting from selling products as a commercial intermediary, by setting the price for the buyer after adding a profit to the factory price, and receiving part or all of the amount before production and delivery (which is done upon request), and after presenting a sample to the buyer, permissible (halal)?

Brokerage (acting as an intermediary between a seller and a buyer) is permissible, and it is permissible to take a commission for it, provided that the party from whom the commission is taken is aware of it, and that the intermediary is not an agent for one of the parties without the permission of his principal. If you are an agent for the factory, you may take a commission on the sale, provided you agree with the factory on a specific price for the commodity, and any amount exceeding that price will be yours without informing the buyer.

However, selling what is not in your possession, such as deceiving the buyer into thinking you own the commodity and then buying it from the factory, is not permissible, due to the hadith (definition): "It is not permissible to combine a loan and a sale, nor two conditions in a sale, nor profit from what has not been guaranteed, nor to sell what is not in your possession."

To resolve this dilemma, the factory can directly sell to the buyer, and you can take a known fee for your mediation. Alternatively, you can agree with the buyer to sell a described commodity in a forward contract (Salam contract) for a known price paid in full at the time of the contract, and then request the commodity from the factory and deliver it to the buyer at the agreed-upon time.