Hi carambola ,
Thank you very much for your post.
Glad to hear you are a long term user.
What I am about to explain will always be the way free ATUM plugin works. We are working on a new premium add-on that will take Purchase Orders to another level, but the release is planned for early 2021.
You can edit any PO and change the amount that you received. Mark it complete and done. If you want to still get the rest of stock from your supplier, create a copy of the same PO and only leave the missing amounts there. You can use the comment section to mention the split.
When the PO is marked complete the items will be added to stock, when you have it in pending stage you will see the items in Stock Central as inbound stock.
Hope this helps.