Item Category with Negative Value in Shipment Cost Document

Dear Gurus,

I am creating a shipment cost document with positive value, and every thing is working fine, system auto create the po and service entry sheet.

but some time due to bad services my client deduct the amount from vendor and it should goes to a seprate GL.

so i create a new item category and assign it with gl as normal, user will enter the penality value in negative, but while save system give the error log.

that over all value is zero or negative so can not be posted.

how i deal with this scenario, how i should post this value so that from over all PO value it should be reduce from PO value.

Please advise at your earliest with thanks in advance.