SAP Note 16741 - Pur. req. from sales document remains open

Symptom:

If a purchase requisition that was generated from a sales document is copied to a purchase order and the quantity is reduced, the remaining purchase requisition quantity remains open.
The purchase requisition can only be copied to one single purchase order items, which means it is not possible to refer to it in more than one purchase order item. If you then only want to order a reduced quantity, the purchase requisition remains open with the remaining quantity. It can only be completed by changing the triggering sales document. Similarly, the sales document itself cannot be completed with a partial order.

Cause and prerequisites

The sales document cannot refer to several purchase orders.

Solution
You should copy the exact purchase requisition quantity to the purchase order item, as a sales document exists for this transaction in the system that was created for that exact quantity.

Key word: ME21/ME22