Short answer is do NOT do it.
The total cost of all inventory type items in QB is held in the account ‘Inventory Asset’ and it might look tempting to just increase or decrease that value by doing a journal entry, after all that seems easier, but it is a bad idea.
Lets say you have 4 widgets in inventory, widget-1, widget-2, widget-3 and widget-4.
And let’s say the stock level and average cost looks like this:
Widget-1 O/H=3 Ave-cost = $12.45
Widget-2 O/H=31 Ave-cost = $2.45
Widget-3 O/H=23 Ave-cost = $1.45
Widget-4 O/H=14 Ave-cost = $9.45
QB multiplies the O/H number by the average cost to get the total value of each item
Widget-1 O/H=3 Ave-cost = $12.45 Total-cost = $37.35
Widget-2 O/H=31 Ave-cost = $2.45 Total-cost = $75.95
Widget-3 O/H=23 Ave-cost = $1.45 Total-cost = $33.35
Widget-4 O/H=14 Ave-cost = $9.45 Total-cost = $132.30
Then QB adds them all up and gets $278.95, and that is the number you see in the account inventory asset.
Basically what QB is doing behind the scenes is carrying a sub-account for each item in inventory. When you buy and sell a widget QB looks to that sub account and changes the item count and cost depending on whether you bought or sold.
IF for whatever reason you decide that the total inventory value should be $275.00 instead of 278.95 and you do a journal entry reducing inventory asset by $3.95 it would look like this.
Debit
Credit Inventory Asset $3.95
And Inventory Asset would have the balance of $275.00.
But QB still carries the costs of each item as before, so when QB adds up the total costs of items in inventory there is a problem, and when you sell the old costs of each item are used. Suppose you sell everything at once, the total cost sent to COGS would be 278.95 and inventory asset would show a Credit balance (negative amount) of $3.95.
All adjustments to inventory need to be made using the adjust inventory icon, if you just need to lower the total cost of inventory for whatever reason, mark it as a value adjustment and lower the total cost of whichever widget needs to be lower. That way QB will add up the new values of each item and arrive at the correct new value of Inventory held in the account Inventory Asset.