Is it permissible for the bank to buy the usurious loan that the questioner took out, by reducing the interest and increasing the fees, bearing in mind that the questioner pays a monthly allowance to his divorced wife?

It is forbidden to borrow with usury (riba), and whoever does so must repent and return what they took, unless they fear harm by not paying the interest. It is not permissible for the bank to buy the debt from you, because that constitutes usury of excess (riba al-fadl) and usury of delay (riba al-nasi'ah). What you described is the forbidden sale of debt, and it could also be a forbidden usurious loan. The "rollover of debt" by extending the term in exchange for increasing the debt is prohibited by Islamic law. Your sale of the debt to the bank is forbidden and a form of usury, and you must repent from it. Monthly expenses do not legitimize forbidden usurious acts.