Russia would move west until Eastern Poland (the Brest Littow Border ) Lwov + the Baltic countries, not Western Prussia (Posen) . I suppose the Germans would want to keep the General Govenrment and the annexed territories. They would probably have to give up some smaller territories such as Pleskau, Memel etc.. but Russia would probably be more interested in taking over Romania and it's oil fields (and blackmail Germany with gas supplies, just in case) and Bulgaria without a fight. I would guess Western Poland would be a buffer zone to prevent new agressions.

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