Sub-contract quantity

We have an sub-contract operation on a job, which is an engineer who bills us by the hour. The job qty is one, but if he bills us for 12 hours, the buyer changes the PO to match 12 hours and does a PO receipt for 12. This causes the qty complete in the operation to 12.

How can we handle this such that the qty complete is only one?

Thanks in advance.

Is it possible to change the unit cost of the one piece on this operation to be whatever the engineer bills you? That way, you close out one piece and the cost for it rolls up correctly.

Yes - we plan to do that. Thanks.