I appreciate you for your prompt response, @lbenioff. Let me provide additional information about the invoice date prior to the bill.

QuickBooks Desktop (QBDT) is adjusting the cost of the item based on the average cost because the average cost is set as the default cost method for inventory items. This is how QuickBooks helps maintain accurate inventory values.

To verify this, you can go to the item's Settings and check the cost method. If it is set to average cost, then QuickBooks will adjust the cost of the item based on the average cost.

As for the issue with the invoice, it is possible that the item's cost was adjusted after the invoice was created, which caused the difference in cost. You may refer to this article for detailed information: Fix negative inventory issues in QBDT. 

The average cost is based on your purchase history for the item, calculated from the beginning to date. QuickBooks will compare the new average cost to the average cost used when selling and purchasing more of the item. Therefore, QuickBooks will make an adjustment entry to both the inventory asset and COGS accounts.

Changing the invoice date will not correct this as the cost of the item has already been adjusted. To prevent this issue in the future, make sure to update the cost of the item before creating any invoices or bills that include it.

Additionally, you might want to refer to this article and learn how to track discrepancies in the Inventory Asset account: Balance Sheet and Inventory/Stock Valuation reports show different amounts for the Inventory Asset a...

I'd be glad to assist if you have additional questions about inventory adjustments in QBDT. Have a good one.