Vendor Bill inventory details handling in API

Hi experts!

We're working on integrating vendor bills from an external system to NetSuite and in need of some insights (or a sanity check ?). Use case in short; lot numbered/serialized items, PO's and receipts recorded in NetSuite, but final Vendor Bills arrive through an external channel. Is the only option to always to draw the vendor bill using the PO as base document in the REST API?

When trying to post the bill with inventory details an error is thrown ("The following serial numbers are already in inventory"), which is basically correct since they were recorded in inventory already. Inventory details are fetched from the fulfillments of course, so they're correct details, but we're not drawing the whole transaction from the base document, rather referring to the existing inventory details. When all data is pulled from the base document everything works fine. This basically leads to situation where in case of data variance between PO/receipt and final Vendor Bill, we would have to first draw the document