What is my duty regarding the unlawful supplementary loan, even though the bank affirmed its permissibility, and what should I do with the amount I have that I have not used for fear of it being unlawful?

You have not clarified the nature of the loan. However, if it is an explicit loan with a stipulated increase, then it is usury (riba). If the transaction is organized tawarruq, where the bank sells the commodity on your behalf, this is forbidden according to most scholars. The transaction might also be a form of the forbidden "revolving debt" (qalb ad-dayn), which involves creating a new debt to pay off the first one, with an increase in the debt and the duration. If the transaction is forbidden and the money is in your possession, then it is obligatory to annul it and return the money to the bank if possible without harm. Otherwise, you may benefit from the money, along with repentance.