Is it permissible or impermissible for a contractor to purchase finishing materials and complete the house with his own money, then add 30% to the cost and collect it in fixed monthly installments?

It is clear from the question that the contractor provides labor and materials, and this is a manufacturing contract (Istisna'). The price must be determined at the time of the contract. There is no objection to agreeing with the contractor on the type of work, specifications, and materials, then the contractor determines the cost and adds their profit, and the contract is made for the total amount, to be paid in monthly installments.