Example:

You have a Trial Balance with $10,000 worth of Inventory on it.

Your opening Inventory:

• 2 x Large Widgets (value $2000 in total - $1,000 per unit)
• 50 x Medium Widgets (value $5000 in total - $100 per unit)
• 300 x Small Widgets (value $3000 in total - $10 per unit)

1. Create a clearing account for your Inventory summary balance (from the Trial Balance) and debit THAT account instead of the inventory current asset account when you import your Trial Balance.

2. Prior to creating the opening Inventory Adjustment, set that clearing account as the Stock Valuation Account (received) account on the Inventory adjustment location:

3. Create and Validate your Inventory adjustment.  You will see, for each product, that the inventory current asset account defined on the category of each product will be used for the debit and the clearing account will be used for the credit. 

If you have a good match between the summary of your inventory from the trial balance and the detail in your adjustment (including accurate costs on each product) then you should see a total of $10,000 in credits that will offset the debit posted to the clearing account during the first step. This will bring the balance of the clearing account to $0.  If it is not $0 you do not have a good match between the summary and detail of your inventory and should resolve the difference until it is not material, then you can expense any remaining amount.

4. Set your inventory adjustment account on both the Incoming and Outgoing Stock Valuation Account fields on the Inventory Adjustment location - this makes sure that all future adjustments correctly account for "lost" and "found" inventory by using the adjustment account instead of the opening inventory clearing account (or the default stock interim (received) account.

In summary:

• debit $10,000 into a clearing account (represents your summary inventory balance) during the import of your opening balances from your Trial Balance.
  
• debit $2,000 + $5,000 + $3,000 into your inventory current asset account via an adjustment (represents the detail of your inventory balance) while crediting $2,000 + $5,000 + $3,000 from the clearing account.

After you are done, there will be nothing in the clearing account, and $10,000 in the inventory current asset account, which came from the inventory adjustment.