Hi Lior,
In my mind this comes way before COPA comes into play. You need to work with SD guy and check whether what exit is available in VF01 / VF02 tcodes. Because this tcode controls whether the Accounting relaase is going to happen or not.
As regards to the Value Field value, I assume it comes form a Condition type(s) from SD billing, therefore that also can be controlled on SD side as well.
Regards,
Dave