1. Standard Cost will be used for the credit, not average or actual.
2. Valuation reports are fully accurate only between Day End Processing and the next posting of Invoices - or when you calculate back to a previous date.

Inventory value is not stored per unit but as a total for all of that item code in any location.
Invoicing changes the quantity of items on hand but not the value. Day End Processing changes the value.

In GridView you can see a "shipped not costed" field that holds the quantity of items invoiced for which day end has not yet been run to change the value.

Therefore, any changes that depend on correct cost value - adjustments, credits to new invoices - should not be done until the value has been changed by "Day End"

Note that "Day End Processing" can be done at any time, though it does require exclusive access and should be preceded by a backup.