Unless the Purchase Order has been issued as, and is stated as, a supplementary agreement to the subcontract then it does not supersede the contract and has no bearing on the subcontract terms and conditions.
I presume that the PO has been issued to comply with your internal procedures, and as you say to facilitate payment, and nothing more.
I presume that the PO has been issued to comply with your internal procedures, and as you say to facilitate payment, and nothing more.