Price Discount

We have the following scenario:

once we enter the Sales Order, then after Sales Update (Status 999) We need to enter the Price Discount against the customer in P4210 Application with Line type P and without Item Code, just Typing a description "Price Discount"in Description1,

it is working but we want to enter the Price discount with Item Code but inventory should not effect only amount should update, Is it Possible?

somebody can help us

Thanks!!!