Issue with Cost of Goods Sold in P&L

My Tax Auditor has just picked out a big issue with my COGS on my P&L.
One item I purchased has increased in price each year for the last few years.
In the first year, lets say 2018, it was say $100 and showed as such on 2018 P&L.
In 2019 it was $200 but shows up as $150 in the 2019 P&L and has changed to $150 in the 2018 P&L.
in 2020 it was $300 but shows up as $200 in the 2020 P&L and has changed the previous years to $200.
I’ve searched the forums but can’t find anything on remedying this. Or am I just doing something wrong?

You could provide some screenshots illustrating the changes in cost of the item as well as the reports that shows the cost treatment/calculations.

Also, you can go to Inventory Items tab and drill down the Total Cost column of one of the inventory item involved.

Take a screenshot of that screen as it will help us identify potential causes for this discrepancy.

@VACUUMDOG, remember that cost of goods sold is calculated in Manager according to the current average cost at the moment the sale is recorded, whether by sales invoice or receipt. Seldom will the cost of goods be equal to their most recent purchase cost, because you probably had some older units still in inventory, dragging down the average cost.

What puzzles me is your statement that the 2018 cost of goods sold has changed to $150 in the 2018 P&L. Cost of goods sold is fixed at time of sale, unless there was an insufficient quantity. Future purchases do not affect it unless they are bringing negative stock levels up. So @Ealfardan is correct; you are going to have to furnish illustrations of the drill-down on the cost balance for inventory items affected.