Hi,
It seems the issue with your supplier SAP. SAP do have the control for delivery based on the PO / SO quantity. As per your explanation, it seems to be a long term agreement with you and the supplier. The supplier has to create a long term sales document, (probably Sales Scheduling Agreements with validity) based on the PO you have given and create schedule lines based on your request to supplier. Refer the thread: "difference between sales order and scheduling ... | SCN and the help doc: Scheduling Agreements - Sales (SD-SLS) - SAP Library
In addition, you have to keep track or amend the PO based on the additional requirements to avoid the invoice block.
Regards,
AKPT