The correct answer is Hungary.
The former Soviet nation that allowed people to flee to Western Germany in 1989 was Hungary.
On September 10, 1989, former Soviet Nation Hungary gives permission to people form East Germany to leave. Yes, the government of Hungary conceded permission to Eastern Germany people to leave to Western Germany. Hungary was part of the Warsaw Pact that strengthen the control of the Soviet Union over Eastern Communist countries. But the government of Hungary decided to break the pact allowing Eastern Germany people to leave to Western Germany. Hungary was supposed to block people from going to the western countries.