I'm not convinced that Stalin did not know about german intentions.
But what choice did he have? At the time of the german attack he kept over 40 his best divisions on the border with China.
Even though Soviet intelegence was reporting that Japanese do not intend to attack from the east, taking those forces from the East was a big gamble.
After all the intelegence might have been right at the time, but once the Germans attack and advance as fast as they did, Japan could have change her mind very easely.
Think of all the resources they could have capturing vast almost unpopulated Siberian territories.