What is the ruling on abandoning the repayment of a loan from an Islamic bank and transferring to an interest-based (ribawi) bank that offers to pay off the loan on one's behalf for half of the original amount, given that financial circumstances have worsened and there is a fear of the Islamic bank foreclosing on the apartment?

It is not permissible to transfer a transaction from an Islamic bank to an interest-based bank, because the interest-based bank will pay off the debt to the Islamic bank and then collect it with interest, which is clear usury (riba). Fear of having the mortgaged apartment seized does not legitimize usury. Rather, one must seek lawful means to pay off the debt, such as entering into a Tawarruq transaction with another Islamic bank to settle the remainder and reduce the installment, or other legitimate alternatives, without falling into prohibited usury.