John,
I agree with the previous two posts, in summary: the Subcon PO is linked to the Routing Operation Sequence not the WO Header, therefore, if you check the PO it refers to the WO and OP Seq. Likewise, if you check the WO Route it refers to the PO at the Subcon Op. This is where the link is IF you are using R31410 to create the PO. If you are not then you can manually maintain a link to the PO through P3112 but I don't think the opposite link can be put into P4310 - correct me if I am wrong but I think this requires a minor mod.
Once this link is established then receiving the PO through P4312 will push the quantity through the Operation in the WO Route. When this is done you can use Superbackflush P31123 or Time Entry P311221 with the "Quantity at Operation verification" processing Option switched on - as per the second reply to your question.
This won't change the Work Order Status so you cannot prevent a WO Completion. However, you couldn't physically complete the WO if the items were not back from Subcon. I have seen a Business where the goods return straight onto the shop from the Vendor without going through P4312 - the GI staff catch up with the receipt days later. This has lead to problems where the WO is completed and then pushed into Accounting prematurely.
This is a procedural issue. Many companies tend to do some basic checking before allowing WO's to be closed for Accounting. Many write simple reports, or even sophisticated UBE's, that will, at the least, show anything not booked, or, at most, check and then change the WO Status based upon hard-coded criteria.