What is the ruling on taking a loan from an Islamic bank that branched off from a usurious bank, and what is the ruling on the murabaha rate (5%) remaining fixed for all cars, and what is the ruling on the murabaha value being waived when the remaining amount is paid?

Dealing with the Islamic branches of riba-based banks is permissible if they adhere to Sharia (definition), and there is no issue with the fixed profit margin in Murabaha. Paying installments in a single lump sum is better to discharge oneself from debt. As for accelerating the payment of a Murabaha debt in exchange for a reduction of part of it (da' wa ta'ajjal - "give up a part and hasten"), this requires elaboration. Murabaha is a deferred sale, not a loan, and if the bank does not purchase the commodity and pay its price on behalf of the client, then it is a forbidden contract.