Table with adds and subtracts, trying to get the balance

Here is the full statement with your code added.

SELECT        i.ItemID AS ID, 
                i.ItemName as Name,
                (SELECT SupplyStatus 
                 FROM SupplyStatus 
                 WHERE SupplyStatusID = i.SupplyStatusID) AS CurrentStatus,
                SUM (
                  CASE  
                  WHEN VolumeOrNumber IS NULL THEN 0 
                  WHEN AdjustmentTypeID = 1 THEN VolumeOrNumber
                  WHEN AdjustmentTypeID = 2 THEN VolumeOrNumber *-1
                  ELSE 0
                END) AS InventoryTotal
FROM Item i                                     
        LEFT JOIN Inventory v ON i.ItemID = v.ItemID
        LEFT JOIN DefaultContainerSizeUnit u ON i.DefaultContainerSizeUnitID = u.DefaultContainerSizeUnitID
WHERE i.CultivationFacilityID = 1       
        AND i.SupplyTypeID = 1                                          
        AND i.ItemTypeID = 1            
        AND (i.EndDate > GETDATE() OR i.EndDate IS NULL)
ORDER BY i.ItemName