Currently, in Xero you can track inventory of an item, and you can get the current stock level of an item using the Items API.
However, you are not able to get the deltas of this using the modified-after header, since even if the stock changes (Via invoices, or adjustments), the Item UpdatedDateUTC is not modified, so you would have to fetch all items and compare within the system.
This effectively means that Xero cannot be considered a source of truth for inventory or stock management, and cannot be relied upon to have correct information.
To add a few details to this.
Currently, in Xero you can track inventory of an item, and you can get the current stock level of an item using the Items API.
However, you are not able to get the deltas of this using the modified-after header, since even if the stock changes (Via invoices, or adjustments), the Item UpdatedDateUTC is not modified, so you would have to fetch all items and compare within the system.
This effectively means that Xero cannot be considered a source of truth for inventory or stock management, and cannot be relied upon to have correct information.