QB Issue Resolution:

Firstly the value of inventory based on the actual costs you have paid for it, and there are only three reasons to adjust your inventory value by itself. Increases in inventory valuation are normally not allowed except in instances outlined under points 1, and 2.

if you are adjusting your records to reflect your year end financial statements which report inventory using a different measurement other than average cost such as LIFO or FIFO.
if you’re a farmer, then your inventory value will likely reflect current market prices
your inventory has significantly devalued due to obsolescence, or significant market devaluation that is not likely to be recoverable.

In the adjust quantity screen select the account you wish to enter the adjustment to Cost of Goods Sold, Inventory Adjustment account, or an other expense account. QuickBooks will automatically make the offsetting adjustment to you Inventory asset account. Select your item to adjust, and enter the new value.

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Edit
Typically in a standard manufacturing setting what would be done is that you would create items for your other inputs, and then create an assembly item for the ‘new’ item; however it also sounds like the materials and costs used in making an item saleable are not standard, and would impact more than one type of expense account making adjustments by the ‘Adjust Inventory ‘ feature difficult.

Enter the purchase and create an inventory part for a new engine (A1) ($200)
when entering bill from or cheques to a vendor to record expenditures to repair or improve an engine ($800)

on the Items tab, select the item (A1) repaired or improved and enter it as a negative quantity and amount, equal to the value currently reported inventory (-$200)
on the items tab select the same inventory part (A1), quantity of one, and the cost equal to the amount deducted from inventory on the previous line plus the amount to be paid to the vendor for repairs or modifications ($200 + 800 = $1,000)

This will clear your previous inventory cost and assign a new total cost to your repaired or modified item, and it will also let you see your conversion costs for a particular item sold, and add any sales taxes paid to its value (if applicable).

Resolution for Issue 'Value adjustment to inventory – only one account?' available: Yes (Solved).
Source: Intuit Community forum.

Value adjustment to inventory – only one account?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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