Absolutely right Mr. Richard, the "Revalue Inventory" is checked, and that's where the whole issue is represented.
I can confirm that further problems could occur with the inventory revaluation, which could be a bit debatable. But the enter/match partial cost modification is a normal behavior that should be well considered by the system. Fortunately, the latest GP 2010 SP4 release included a hot fix for the issue described above.
Thanks,
Your feedback is highly appreciated.