Is buying cars from a dealership with a bank loan, then selling them to the same dealership at a lower price, and repaying the bank an amount with an increase (500,000 + 40.5%) considered usury (riba)?

If what the questioner is doing is taking a check from the bank for a certain amount on condition of returning it with an increase, then this is usury (riba), which is forbidden by the consensus of Muslims. As for the Murabaha sale conducted by Islamic banks, it is permissible under specific conditions.