JAPAN has discovered just what a $2.5 billion aid package will buy from Moscow.The package, announced last week, was not officially linked to Japan's demand that the Soviet Union return four small, wind-swept islands the Soviets have held since the end of World War II. Still, the linkage was made clear enough when leaders in Moscow made several concessions this week during a visit by Japanese Foreign Minister Taro Nakayama. "Of course, we should not talk about this [linkage] in public," says Seizaburo Sato, a Tokyo University professor and foreign policy expert. "While the Russians do need money, they also have their pride." In one concession, Soviet President Mikhail Gorbachev hinted that Japan had a claim to two of the four islands when he appeared to recognize a 1956 joint declaration with Japan. That document was disavowed by Moscow in 1960. In a second move, the Soviet Union also announced that it would withdraw about 30 percent of the more than 7,000 troops on the Kurile Islands (known as the Northern Territories in Japan), with more withdrawals possible. "It's a good thing," responds Foreign Ministry spokesman Taizo Watanabe. "But there should be no forces there." And thirdly, after refusing in the past to discuss the islands in official talks, Moscow agreed to set up a separate forum on the dispute. But perhaps the biggest prize for Japan is that it has found a more willing negotiator in the Russian Republic, whose territory includes the disputed Kurile islands. "It is a victory for Japan that the Russian Republic is the major negotiator," says Dr. Sato. "We are now are at the starting point of negotiations. The islands will come back to Japan pretty quickly." Still, Mr. Nakayama, who visited Moscow from Oct. 11-17, warned that neither the Soviet central government nor the Russian Republic is prepared to give up sovereignty over the islands. Soviet officials, however, appeared to have moved the dispute from a legal argument over historical claims to a question of how to convince Russian public opinion to accept Japanese control of the islands. Residents on the islands voted overwhelmingly last March against Japan taking over. In early October, the Oriental Studies Institute in Moscow revealed an 1853 letter from Czar Nicholas I that admitted Japan's sovereignty over the four islands - Habomai, Shikotan, Etorofu, and Kunashiri. Before that, the acting speaker of the Russian parliament, Ruslan Khasbulatov, suggested talks with Japan should not be based on who won the war. Moscow has claimed the islands as spoils for joining the Pacific war just before it ended. After winning direct talks with the Russian Republic during his trip, Nakayama invited President Boris Yeltsin to visit Japan sometime next year. That trip, or an expected trip by the Japanese prime minister to Moscow, may serve as the next opportunity for a big push on the islands issue. Japan's aid package, which roughly matched those of the United States and the European Community, included $500 million in humanitarian aid and $200 million in trade credit for exports from Russia to Japan. The largest part, however ($1.8 billion in government insurance to cover any trade losses by Japanese firms), is not direct aid. Japan has refused large financial aid until the islands are returned. Confident that the issue was moving toward a solution, Japan also prepared a transition program for the islands' 24,000 Russian residents who are over 18 years of age. "The inhabitants are probably concerned about the type of rights they will enjoy after the islands are returned," Nakayama said before his trip. The government plan would give them at least three benefits: temporary resident status; property insurance and repatriation allowances for those who want to leave; and a chance to be either a Japanese citizen or permanent resident.