and the Soviet Union was visible, for example, in the four Gestapo-NKVD Conferences, where the occupying powers discussed plans for dealing with the Polish resistance movement, for the further destruction of Poland, and which enabled both parties to exchange Polish prisoners of interest prior to the signing of German–Soviet Frontier Treaty in Moscow in the presence of Joseph Stalin. The cooperation between Gestapo and NKVD continued, resulting in further exchanges of prisoners, among them Margarete Buber-Neumann, Alexander Weissberg-Cybulski, Betty Olberg and Max Zucker. Amendment of the Secret Protocols On September 25, when Hitler was still going to proceed to Lithuania, the Soviet Union proposed to renegotiate the spheres of interest. On September 28, 1939, in Moscow Molotov and Ribbentrop signed the German-Soviet Boundary and Friendship Treaty, determining the boundary of their respective national interests in the territory of the former Polish state. In a secret supplementary protocol to the treaty the spheres of interest outside Poland were renegotiated, and in exchange for some already captured portions of the Polish territory Germany acknowledged still independent Lithuania part of the Soviet zone. Expanded commercial pact Germany and the Soviet Union entered an intricate trade pact on February 11, 1940, that was over four times larger than the one the two countries had signed in August 1939. The trade pact helped Germany to surmount a British blockade of Germany. In the first year, Germany received one million tons of cereals, half a million tons of wheat, 900,000 tons of oil, 100,000 tons of cotton, 500,000 tons of phosphates and considerable amounts of other vital raw materials, along with the transit of one million tons of soybeans from Manchuria. These and other supplies were being transported through Soviet and occupied Polish territories, and this allowed Nazi Germany to circumvent the British naval blockade. The Soviets were to receive a naval cruiser, the plans to the battleship Bismarck, heavy naval guns, other naval gear and thirty of Germany's latest warplanes, including the Bf 109 fighter, Bf 110 fighter and Ju 88 bomber. The Soviets would also receive oil and electric equipment, locomotives, turbines, generators, diesel engines, ships, machine tools and samples of Germany artillery, tanks, explosives, chemical-warfare equipment and other items. The Soviets also helped Germany to avoid British naval blockades by providing a submarine base, Basis Nord, in the northern Soviet Union near Murmansk. This also provided a refueling and maintenance location, and a