Russia had the largest number of casualties during WW2, and Russia had taken many Japanese prisoners. So, why didn't Russia receive a reparation from Japan to help boost Russia's economy after the war?
Japan agreed to pay war reparations of 1.3 trillion yen. The Japanese GDP in 1952 was 6,217 billion yen. So the reparation was 20.91% of the Japanese GDP. The Japanese GDP in 2011 was equivalent to $5.869 trillion 2011 USD. So the reparations were equivalent to 1,224 billion 2011 USD. This was all proposed at the Treaty of San Francisco in 1952. The soviet Union, however, disagreed with many points:
that Communist China was not invited to participate despite being one of the main victims of the Japanese aggression
that the Soviet Union was not properly consulted when the treaty was being prepared
that the treaty sets up Japan as an American military base and draws Japan into a military coalition directed against the Soviet Union
that the treaty was in effect a separate peace treaty
that the draft treaty violated the rights of China to Taiwan and several other islands
that several Japanese islands were ceded by the treaty to the United States despite the U.S. not having any legitimate claim to them
that the draft treaty, in violation of the Yalta agreement, did not recognize the Soviet Union's sovereignty over South Sakhalin and the Kuril Islands
They also objected over other minor points. In the end, the Soviet Union refused to sign along with Czechoslovakia and Poland.
From the start of the conference the Soviet Union expressed vigorous and vocal opposition to the draft treaty text prepared by the United States and the United Kingdom. The Soviet delegation made several unsuccessful procedural attempts to stall the proceedings.
It was not until October 19, 1956, that Japan and the Soviet Union signed a Joint Declaration ending the war and reestablishing diplomatic relations.
Sakhalin was first divided between Russia and Japan by the treaty of Shimoda in 1855. Then by the treaty of St.Petersburg in 1875 the Japanese half was peacefully exchanged for Kuril islands - so Sakhalin became fully Russian. Then (again!) Sakhalin was divvied up (by force, so to speak) as a result of Russo-Japanese war in 1905, with Japan taking the lower half. And then in August 1945 that half of Sakhalin was taken by Soviet army, and then officially ceded back to USSR by the S.F. treaty.