Hi
Wow! Very challenging issue! Especially not finding it for 2 months. The only steps I think you may try is as follows:
1) Post Item Journal Postive Quantity and correct cost with correct date to 31/12/2020
2) Go to Item Card - Click More Options
3) Click Related - History - Entries - Item Application Worksheet - Select Positive Adjustment posted on 31/12/7899 - select Related > View Applied Entries. Select Remove Application for all Negative Qty (Outbound Item Ledger Entries) applied to this wrong Posting Date
4) Click Actions > Functions > Reapply - this should now reapply these Negative entries to the 31/12/2020 correct ILE Positive Adjustment from step 1)
5) Enter an Item Journal Negative Entry with the Posting Date on 31/12/7899 for same quantity as the Positive Adjustment posted on 31/12/7899 - Set Applies-to Entry to the Positive Adjustment ILE posted on 31/12/7899. This should fix-apply the negative to the positive to clear the wrongly posted cost and entries on the future date completely.
I hope the steps enable you to work through the issue, which appears to be very challenging.
After completing above steps, please update General Ledger Setup > Allow Posting From Date and Allow Posting To Date to better control the posting of entries going forward. The User Setup also includes the applicable date range.
