Debit Note & Excise part

dear

                     proportionate duty to be debited. as you received (rate of duty) same duty to be debited. no question of current rate of duty or any. just to pass on duty.

In this scenario you are returning the duty credit availed as the goods are returned and not utilized. So it will go back as much as you have availed. Nothing to do with the current or any other prevailing rate.

Dear, the basic thing is that you have not utilised the duty amount. So, you have to pay the duty amouount as you have taken credit.