Thank you very much for your post.
ATUM includes the Purchase Orders module, where you can get complete control over the tracking of supplier's inventory with the following statuses:
Pending: When create the PO but is not sent. All the products within these POs will show in the Inbound Stock list and reflect their totals in Stock Central.
Ordered: When the PO is sent to the supplier (The products will keep in the Inbound Stock list).
On the way in: When products are on the way (The products will keep in the Inbound Stock list).
Receiving: When products were delivered but have not been registered yet (The products will keep in the Inbound Stock list).
Received: When products were delivered and registered in the warehouse. When this status is activated the products will be added to your stock automatically and are no longer listed in Inbound Stock list and their totals not reflected in Stock Central’s Inbound Stock column.
ATUM also includes the Inbound Stock module where you can track the stock of the Purchase Orders.
You can read more about ATUM Purchase Orders here.
If you are planning to use Multi-Inventory premium add-on, it's fully compatible with the ATUM Purchase Orders system. You can read more about Multi-Inventory records in Purchase Orders here.
Feel free to create your demo site for Multi-Inventory and check if it meets all your requirements. You can create the demo site here.
I hope this helps,