exise duty entry

hi,

Good morning to all MMGURUS.here i am facing one issue from so many days.the end user prepare a po for a material with out material no.i.e by description he create a po. while creating po he dont know that this material having excise duty.according to contract a down payment of 30% of the payment done against po.now invoice came with exsice duty . according to my knowledge for entry of excise duty, material code req. now i have create a material code for this .now when i made change to existing po it wont accept as it is having down payment .i cant delete line item earlier created is there any solution for this .immediate response highly appreciated and awards also

regards

srinivas kona.