What is the ruling on interest whose percentage is not specified and is left to the borrower, and who pays zakat on this money, the lender or the borrower?

It is not permissible to borrow with interest, because it is prohibited usury (riba). A loan is a contract of donation, and it is not permissible to take a benefit in return for it. The Prophet, peace and blessings be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses. It is permissible for the borrower to add something upon repayment as a voluntary act, not as a condition. The zakat on a debt is obligatory upon the lender, not the borrower.