why would Stalin consider selling out Germany ITTL they have ongoing conflict against Japan? wouldn't the Soviet fear be that after selling out the Nazis the successor regime would join in with Allies against USSR? (that was their fear WITHOUT conflict in the East)
Because Stalin had no love for Hitler and a clearer idea than most about Hitler's actual beliefs. If the price is right, and the deal looks trustworthy enough, Stalin will stab Hitler in the back or in the face.
Stalin wasn't just a passive participant in WW2, and he wouldn't be passive in TTL. He'd be careful, paranoid and ruthless, to be sure. But he would act.
fasquardon