Japan had two border incidents (Lake Khasan and Nomonhan). Japan was squashed on both occassions which is why Japan didn't DoW the Soviets after the kick off of Operation Barbarossa. You could have one of those escalate. IOTL Stalin didn't let them escalate because he was wary of Nazi-Germany since everyone in both Moscow and Berlin knew that war was inevitable, it was just a matter of who striked first. Stalin didn't want to denude the western border. If there's no German threat he will order the Red Army to steamroller the Japanese in Manchuria and get involved in a 2-3 year war, leading to an entirely communist Korean peninsula and an earlier victory in the Chinese Civil War for Mao Zedong.
You'd have to change the German leadership to one that can satisfy itself with a state of Cold War with the Soviets, watching it collapse economically and puppetizing the eastern European states and dominating them economically and organizing them into something a defence/economic block, a combination of the EU and NATO (without the US of course) or a regime that's friendly to the USSR, although that would be hard to get as conservatism and anti-communism were both deeply entrenched in Germany after 1936.
This way Stalin can risk exposing his western frontier without inciting German aggression.