Is purchasing a computer in installments through a bank that pays its price to the seller and takes installments with a 1.5% increase considered usury (riba) and forbidden, given that banks in Tunisia are usurious and there are no Islamic banks?

Dealing with interest-based banks is not permissible, unless a Muslim cannot find an Islamic bank that fulfills their needs. In cases of necessity, it is permissible to resort to a bank for the purchase of a computer, provided that the transaction is valid according to Sharia (definition) regulations.

There are two permissible forms of installment sales: 1. The commodity is owned by the bank and is in its possession, and the buyer purchases it from the bank at a known price that is not subject to increase. 2. The buyer requests the bank to purchase the commodity from a third party. If the bank purchases it and it enters into its possession and guarantee, the buyer then purchases it from the bank at a price known to both parties and not subject to increase in case of delayed payment.

However, what banks do by paying the price on behalf of the client and recovering it with an increase is a forbidden interest-based loan. The computer must enter into the bank's possession and ownership.