When we change either the final invoice or goods receipt fields for an ordered PO, the system resends the PO to the vendor. In our case it's an automated XML process so it's preventing us from setting these fields which need to be set to release the funds in the backend system.
I'm not sure if this is a problem or something we need to code around via enhancements.
We've looked at table BBPD_PO_METAOUT but that doesn't seem to have any influence as in our case the code to read the table doesn't seem to be executed. txn BBP_PO_ACTION_CONF also doesn't seem relevant in this case.