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you would need to do this both relationally and calculatory.
Set up a relationship between Products and Invoice Line Items
In Products, have a numeric field of "initial inventory"
In Products, have a calc field of "counted used", that uses count(Invoice Line Items::quantity)
In Products, have a calc field of "remaining inventory", that uses initial inventory - counted used.
This will give you a field in products that shows the amount of product remaining.