(Map taken from abother thread)
USSR takes Mongolia (all of them) as a single puppet (blue), Red directly to the USSR, the yellow bits excluding Japan and Japanese occupied Korea is economic zone where the USSR and China work together. Manchuria was the engine of China's rise and there is no reason for the USSR to prop them up. Trade with them was minimal and they are a big competitor. Owning the base of their rise will make them much more controllable. They can't decline, they don't have the capability to do so.
OTL most of China - especially the urban areas - was ruled by KMT at this time, who are our enemies, while the Chinese Soviet Party owned the rural areas. Helping the Chinese Soviets to build their industrial base and provide high value food production will establish a base to have a better defence against the Japanese, who OTL invaded the area from their Korean puppet in 1931. Or worst case, bring in the Japanese ourselves to the yellow area to keep us safe for longer. Then kick them out together with the Chinese Soviets post WW2 like OTL (but keep the red & blue bits), just give the Chinese Korea as a puppet. Its very historical anyway.