When making a stock adjustment to increase the level of an item, it is not required to enter a Total Cost if no cost was incurred. In that case, you can simply enter 0 under Total Cost.
Here in the Material History, you can see:

Transaction:
Resulting Stock:
The New Average Cost (also referred to as the Unit Cost) represents the average cost per item, which can include shipping, freight, or other delivery-related costs. So, if you didn’t purchase these items, the increase in stock will lower your average cost.
We hope this helps clarify how stock adjustments work.
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