Part 1 can only be canceled along with Material document cancellation. Since you performed return delivery, now there is no solution to correct it.
I can understand the issue. Once UD is taken, you can not cancel the Material document. You should have implemented UD reversal note and tried cancelling the Material document instead of Return delivery.
There is no harm if you do not cancel the Excise invoice since Part 2 posting is not yet done. If you wish you do it still, change the Excise invoice status to "B" Canceled at table level.