Is it permissible to donate the remaining installment amount of a loan to the poor if the court rules in favor of the borrower, despite his 20% doubt about not having repaid it, and the bank having no material proof of non-repayment?

Repentance is obligatory for engaging in interest-based borrowing. If the installment demanded exceeds the principal amount, it is not obligatory to pay it because it is usury. However, if the installment is part of the principal amount and the bank claims non-payment, then its statement is to be accepted because the default is non-receipt. You must provide evidence of payment. If the debt is proven against you and you fail to provide evidence, then a judge's ruling to the contrary will not benefit you. You must pay the money to the bank, and it is not permissible to give it away as charity.