All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
We will summarize our answer to your question in the following points:
Firstly: There is no harm in benefiting from and making use of the products and items you had purchased from the bank.
Secondly: you did not clarify to us the method of the transaction between you and the bank. If we presume that the bank sold you the goods after possessing them, then there is nothing wrong in that, even if the price of the sold goods was more than its current price, because time has a impact on the price; immediately paying for the sale is not like delaying the payment.
If the bank did not own the goods, and it just paid the price to the seller on your behalf, and then the bank got the money from you with additional amount, then this is actually a Riba-based loan. However, since you had undertaken this transaction while you were unaware of the ruling (that it is forbidden), then we hope that you are not sinful.
Thirdly: The fact that the bank wants to drop some of te interest amount if you hasten the repayment, then this is a good thing and you should hasten to repay. This is because if we presume that the transaction is Riba (interest and/or usury), then you are not obliged to pay the interest anyways. You should seek a means to drop the repayment of this interest.
If we presume that the transaction was a valid and permissible sale transaction and you wanted to hasten in repaying, and for this reason, the bank dropped some interests, then this is acceptable.
For more benefit, please refer to Fataawa 95608 and 12927.
Allah knows best.