This is a really important but fiendishily complicated add-on proposed. For us, important things would include the following workflow:
1. On planning the purchase, we don't know whether the stock will be a different batch or cost price, so a new inventory is not created immediately on PO creation
2. When ordering, the actual price, including discounts or shipping fees, is locked in and Batch/BB date may be known, so when updating the PO, these fields will be available to fill optionally, and new inventories are created automatically if the data in them does not match one of the previous inventories.
3. When stock arrives, the price, batch & BB may be known, so we need another opportunity to update the PO.
4. Some lines may have exceptions, such as missing or damaged, so there needs to be the ability to put into stock some but not all lines.
5. When the invoice arrives, there may again be the need to adjust cost.
6. All adjustments and comments to the PO from creation need to be logged, until the final PO is signed off.