Question

What did Stalin agree to do in return for the Allied invasion of France?

Answer 1

Attack from the east

Answer 2

At the Tehran Conference (November 1943), Stalin agreed that the Soviet Union would enter the war against Japan once Nazi Germany was defeated.
http://en.wikipedia.org/wiki/Soviet%E2%80%93Japanese_War_(1945)

Answer 3

The U.S.S.R. agreed to stage a final offensive against the Nazi forces in Eastern Europe as long as the United States, Canada, and many other nations invaded France. Stalin realized the Nazi's were too powerful for the Russian people to fight on their own, but after victories at Stalingrad and Moscow, the German's were put on the defensive. Stalin later agreed to enter the war against the Japanese Empire with the United States after Germany was defeated and Hitler was dead.