To be accurate, Roosevelt only asked for Soviet help against Japan at Yalta because he wasn't sure at that stage if the nukes would work. The Japanese knew they faced inihilation after Nagasaki proved to them Hiroshima wasn't a one off.
Stalin was a cagey f
er. He grabbed as much as he could while it was there for the pickings (Sakhalin etc)
I think you'll find though that the claim "that the Russian campaign in Manchuria was the reason that the Japanese surrendered" is a little myopic.