Is it permissible to buy a house from a friend in this manner: paying him a portion in cash, and completing the payment of the remaining installments to the bank with the property mortgaged to the bank until all installments are paid?

If the bank is usurious, then it is not permissible to borrow from it or pay usurious interest to it. However, if it is not usurious, the matter divides into two cases:

1. If your friend purchased the house from the bank according to the Shariah-compliant regulations for installment sales, there is no harm in your purchasing it by mortgaging the house to the bank until the price is paid.

2. If your friend purchased the house with a contract that includes a Shariah violation (such as a late payment penalty), then the ruling on purchasing the house in this case is based on the difference of opinion among scholars regarding the ruling on what is acquired through a فسخ (fasakh) or voidable contract. Some of them hold that it is not permissible to dispose of it and that it must be returned, while others believe that a فسخ (fasakh) or voidable transaction conveys ownership through authorized possession.