Shiva,
It was nice to read your post! You conveyed some subtle differences in the issue and the note.
The note talks about possible issues (in 3rd party sale scenarios) if changes are done in ME22N (or in the change mode in the PO).
But still, always I have understood that the billing quantity (to external customer) depend upon the IV quantity (not PO quantity etc.).
I have requested the OP to test, the changes in the sales order of the 3rd party sales (both decrease and increase the quantity) without using t-code ME22N and check if the above error occurs.