The technical reason why the USSR allowed Truman to send U.S. troops
to Korea in 1950 was that the Soviet representative on the Security
Counsel, Mr. Malik, was boycotting the U.N. over its refusal to
recognize communist China. Therefore, he was absent when the decision
to send U.N. troops was taken.
"Military historian John S.D. Eisenhower on the Korean War" (June 19,
"Lashmar, Paul, 'Stalin's "Hot" War,' New Statesman & Society, vol. 9,
no. 388, Feburary 2, 1996"
Mount Holyoke College: International Relations Program
The behind-the-scenes reasons why Stalin did not get involved are
described on the second page I have cited. Basically, Stalin did not
want to provoke the Americans at that time, allowed North Korea to
invade South Korea only when convinced that the conflict would be
quick and not involve the Americans, and made sure to keep Soviet
participation to a minimum when the Americans did get involved.
I hope that this information is helpful.
Search terms used on Google:
"security counsel" "korean war" soviet
[I tried several other searches as well, but this search resulted in
the pages I have cited.]