NEC ECS: Subcontract vs Purchase Order

We have a signed Subcontract in place with a Subcontractor that lays out everything in plain English and is pretty straight forward, it has X5 & X7 in there with the dates and values laid out. We have also issued a Purchase Order to the Subcontractor as it is required as a method for payment. The Purchase Order has separate provisions within it for recouping any ‘liabilities incurred due to the Subcontractor’. If they were to have conflicting views on damages which would take precedence? I always assume a Subcontract stands above a Purchase Order as it is signed by both parties but a Purchase Order is surely acknowledged once they have invoiced against this?

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.