Such a move would be unlikely, as Stalin was not suicidal. Such a move would be just that, in the meaning it would destroy the Red Army on foreign ground, when the Germans countered the attack, by shifting some divissions from the West to the East by their elaborate railsystem. This capacity is not to be underestimated, as it technically meant that the German Army could transport several armycorpses in only a matter of days from West to East, if needed that badly.
For the War in the West, it would mean only a slight delay, since the remainign German Armies there would still be able to crush the western Allies, although perhaps a bit slower, due to their now more limmited numbers on the ground and in the air, but still far more powerfull units remained to fight off the Western Allies, who were inferiour in quality and equipment.
The Red Army would likely be succesfull in capturing part of German occupied Polland, but not much more, given its lack of mobility in 1940. The more mobile Germans would encircle and crush the Russians, the way they did in 1941. The result would be an early on assault on the now severely weakened USSR, who was only just starting to shift military industy to east of the Ural and was therefore very badly situated to fight already in 1940. Without this industry, which could already be destroyed in 1940 by the advancing Germans, if the scenario as mentioned unfolded, the USSR was technocally incapacitated adn unable to hold itself against even a modest German assault.