What is the ruling on paying only the principal amount to a usurious bank, or paying the agreed-upon amount without adhering to the monthly installment and deadlines, and what is the expiation for this sin?

Allah has forbidden usury and made it one of the grave sins, threatening those who deal in it with war and the removal of blessings. Whoever takes out a usurious loan and then repents, is only obligated to return the principal without the usurious interest. It is neither permissible for the lender to take it, nor for the borrower to pay it willingly.