Hello,
Since PO Value is one of the characteristics defined in your release strategy and in case of Consignment, STO, Return POs the PO Value field is blank / NA. So what you can do is you can create a new release stategry: Exclude the PO value characteristic and include the PO document type characteristic in the new classs and assign the PO order types where the PO value is blank / NA.
Regards,
Pavan