Tax calculation only on base price

Hi,

I have a requirement, for a material as a base price we want to add base price in two consitions

e.g base price is 1000

In PO

PB00 : 900

ZPBX : 100,

so my total amount will be 1000 but the tax calculation should be only on PB00 amount not on the PB00 + ZPBX , how we can get this.

re