What is the ruling on buying a house from its owner, knowing that the buyer will complete the payment of the remaining price of the house to a housing association or a bank that the seller was obligated to pay?

There is no harm in purchasing a dwelling from its owner if the transaction is legitimate, even if the owner acquired the dwelling through an interest-based loan. This is because the impermissibility of the loan pertains to his personal liability and not to the dwelling itself. There is also no objection to paying the price to the association or the bank, provided that there is no commitment to pay usurious interest if payments are delayed.