Is it permissible to buy a house from a relative who owns this house and pays its installments to a usurious bank, on the condition that the amount paid by the buyer is used to settle the remaining usurious installments?

If the intention is that your relative owns the house and wants to sell it to you, while keeping it mortgaged with him for the remaining price until he receives it in full, then there is no objection to that, according to the preponderant opinion, and the house becomes your property. It states in a resolution by the Islamic Fiqh Academy that the seller is not entitled to retain ownership of the sold item after the sale. However, he may stipulate a mortgage of the sold item with him to guarantee his right to collect the deferred installments. As for your relative buying the house with an interest-based loan and wanting to sell it to repay the loan installments, that does not prevent dealing with him and buying the house from him.