Overwrite Tax Code in Invoice tab of PO line item

Hi all.

Is there any user exit through which I can Overwrite Tax Code in Invoice tab of PO line item.

I dont think a field exit will help since I have decide the tax code value based on the line item details like plant and a/c assignment cat. and header Purchase org.

Any help will be very much appreciated.

Thanks in anticipation.

Regards,

Aditya