Horse trading - leasehold in Siberia in return for purchasing Russian commodities and mil HW.... the mother of all kick backs. Also, if PLA secure Siberia, then this solves a key logistical problem for Moscow and frees Moscow to thrust West ward and South ward with ease. Witness WW2 - Stalin was concerned that the Japanese would turn attention northward but was quite relieved at the Japanese focus on China and SE Asia, and even more relieved when Japan had to fight the US in the S. Pacific. If Russia can be assured that there will effectively be no Eastern front then Europe and the Middle East are hers.
However, in my series, Russia's dreams of wealth and neutrality are shattered by a Barbarosa II...coming form the east.
Japan loose several major battle to Zukov in 1939. Also Stalin through spies know Japan will not invade. As for horse trading, maybe Americans ready to lease out America land, but Russians not. Already new development and recolonization effort along Chinese border. As for thrusting, Russia is more to fear NATO/EU aggressors. It is NATO who bomb Christians for Islamics over and over...well Orthodox Christians...then stand and make postcards while Christian churches burn and give hugs and kisses to Islamics.