Hi Manuel,
I wrote my previous reply (Nov 27, 2014 6:04 PM) without reading this latest conversation.
For reversals, there can be another reason for difference postings - the system always tries to offset the original quantity + amount, but if there is insufficient value then MAP (moving average price) becomes zero and the rest goes to AUM - similar to the example I showed for returns.
It is another rule in standard SAP that you cannot have positive stock with negative value or vice versa, so as soon as this is going to happen (with reversals, returns etc.) then MAP becomes zero and there is a booking to a difference account.
Summary - you surely need to configure this account. Else, any exceptions like this will lead to failed postings.
Good luck!