I solved it by following these steps provided by EPiServer:
The unfortunate thing is that the in stock quantity for a line item in the cart is set when running a workflow (CartPrepare) and will not be changed if you change the inventory level of the warehouse. You can force the change by removing the line item from the shipment and adding it again. The new line item will have the new inventory level and you should be able to send it to the warehouse.
If you want to change how Commerce is managing this you can change what the Shipment Release handler is in the C:\EPiServer\EPiServerCommerceManager2\eCommerceFramework\5.3\Shared\Apps\Order\Config\View\Forms\PurchaseOrder-ObjectView.OrderShipment.xml. Reflect the default one Mediachase.Commerce.Manager.Order.CommandHandlers.PurchaseOrderHandlers.ShipmentReleaseHandler and modify it and point to your modified one in the xml.
I'm facing a very weird issue when trying to send a purchase to the Warehouse. It displays the following error message when pressing the "Send to Warehouse" button (once payments are placed and rest of stuff):
150 units were ordered for that product. So yes, I went to the Catalog Manager, and indeed I found that the stock value for that product was set at 51 In Stock units at the Warehouse Inventory tab. As we required that our system must ignore any stock tracking (products must be always available and orders must be processed regardless of stock), I setup the following options for this product:
1- Pricing tab -> Track Inventory to "No"
2- Warehouse Inventory tab -> Inventory Status to "Disabled"
3- In Stock -> I think it shouldn't be required with the previous options, but I set it up from 51 to 1000.
Afterwards, I tried to press the "Send to Warehouse" button again for that purchase, but I still receive the same exact error, with the "Only 51 can be shipped" (although I already changed that number, so I don't know where it comes from...)
Does anyone have any clue about why does this happen and how can I solve it?
Thank you and kind regards,