Is it permissible to purchase a house from a seller who owes a debt to the bank, knowing that the price of the house is agreed upon, the installment payments are specified, there are no additional interests or fees, and the seller legally owns the house?

There is no harm in buying a house from its owner, even if he bought it with an interest-based loan or owes debt to the bank, as long as he is not prohibited from selling it until he pays off his debt. The borrower who engages in usury commits a grave sin, but the borrowed money and the house built with it become his property, and its sale is valid. The Messenger of Allah, peace and blessings be upon him, engaged in buying and selling with the Jews, even though they dealt in usury.