First off, sounds like you're dealing with a classic inventory valuation mess-up. This kind of error in settings can really sneak up on ya, especially with FIFO valuations. Number one thing is to fix that specific inventory item in quickbooks online to get your costs tracking right.

I'd start by checking the audit trail for the original adjustment. yeah, you mentioned it was made years ago, but trackin' down the transaction is key to seeing what was changed. Once you spot it, if possible, reverse that entry or do a correcting entry to specifically adjust the item's cost. I get why you don’t wanna mess with COGS through journal entries - it don't fix the inventory valuation issue, just tweaks the financial reports temporarily.

To really sort this out, adjust the cost of this particular inventory item directly in qbo. Head to the "products and services" list, find the item, and manually adjust its cost. Double-check with your accounting partner or team first - they might wanna make extra offsetting entries for reporting periods. This way, your future sales pull the right cost amount.

Tackling old transaction histories can be tricky, but once you get that outlier aligned with the right value, FIFO calcs in QB should sort out. trusting your instincts is just as necessary tho. And yeah, be sure to back up your data before making big changes!