Ok, let's say that you change the Key field of Temp to Part #.
Make a new formula text field in Inventory called [Part# mirror for relationship] with a formula of [Part #]
Then make a relationship where one Temp has Many inventory based on that field,
Then make a summary checkbox field on the relationship if there are Any Inventory records.
ok, so now we have a checkbox field on Temp that will be unchecked if the Temp part has no corresponding inventory part.
So now actually, you can make a simple two step Pipeline to trigger when a Temp is created and it has no Inventory, and the next step will be to create an inventory record.
Feel free post back if you have questions.