In early April 1927, Greim and his family left Canton, taking the Trans–Siberian Railway via Harbin, Moscow and Warsaw, arriving in Munich on 1 May 1927. In Moscow, Greim failed to help a longtime school and war friend, Josef Römer, with establishing contact to the Soviets. Römer, who was a communist and member of the German resistance to Nazism, was later imprisoned in 1934 in the Dachau concentration camp. In 1939, Greim was instrumental in getting Römer released from prison. When Römer was again arrested and tried before the People's Court () on 19 June 1944, Greim was unable to help further, Römer was sentenced to death and executed on 25 September 1944.
