From Tooze, The Wages of Destruction, "When Goebbels visited... Rastenburg on 18 August 1941, he was shocked to fund his Fuehrer talking of a negotiated peace with Stalin."
From Montefiore, The Court of the Red Tsar, "Around the time of Yakov's capture, Stalin made his first approach to Hitler... whether he would be satisfied with the Ukraine, Belorussia, Moldova, and the Baltics, a second Brest-Litovsk?" (this particular message was not sent to Germany by the intermediary, but gives an idea of what the Soviets would have given around this time)
So what if after the Battle of Kiev, say, Hitler had followed through on the idea of a negotiated peace, sounded out the USSR, gotten, and accepted the above offer? Would he still declare war on the US? How would the Battle of the Atlantic have been affected, and what would the wider strategic implications of such an agreement be?