Want to change accouting document line item at the time of PGI

lHi all experts,

At the time of PGI system generate material document and than generate accouting document.

Now i want to change one ine item of dicount GL account for that accounting document.

Field name AUFNR.

Right now system will take internal order against that specific discount GL.

I want to remove clear that field from accounting document.

Anyone suggest me userexit or BADI through which i will achieve the same.

Its Urgent..

Regards,

Chirag