Is the debtor obligated to return the value of the carton at the price at the time of borrowing, or at the price at the time of repayment, knowing that it was agreed to return it as paper of the same type?

This transaction is a loan, not a borrowing. A loan entails transferring ownership of an item in exchange for its equivalent. A loan is permissible for anything that can be sold by a Salam contract. It is obligatory to return the like for fungible items. If both parties agree to return the value, it is permissible, provided that it is at the price of that day, to avoid profiting from what has not been guaranteed, as the Prophet prohibited profiting from what has not been guaranteed. Selling a debt to the debtor for an increase leads to a loan that brings a benefit, which is forbidden.