Have DDR keep the land east of the Oder-Neisse line that belonged to Germany after WW1. Land that is also contiguous with OTL DDR, thus meaning that East Prussia is not part of this ATL DDR.
Incidentally, the SED did not recognize the Oder-western Neisse line until 1949 with the proclamation of the DDR. I do not know whether before then they really thought that Stalin might modify the border or whether they simply were trying to appeal to public opinion in the western zones by keeping up the hope that if only Germany went Communist, it could get better borders.