What is the ruling on purchasing building materials in installments from a person who directs me to a merchant and handles his account, then I pay him back with an increase, or dealing with a contractor who owns building materials and trades in them, or undertakes all construction costs and sets the price per meter?

The first scenario is an interest-based loan (riba) because the person pays the price on behalf of the enquirer and then recovers it from him with a guaranteed increase. As for the second scenario, there is no objection to it. It is a type of trade that can be regulated according to the Istisna' contract, whereby the contractor sells the materials on credit for more than their immediate price, and the lease (ijara) is included with the sale of the materials.