There was never any agreement, because a suggestion of even such agreement is anachronistic garbage. I'll let Soviet foreign minister Shevardnadze explain it:

There was never any agreement, because a suggestion of even such agreement is anachronistic garbage. I'll let Soviet foreign minister Shevardnadze explain it: >> SPIEGEL ONLINE: In February 1990, Germany's foreign minister at the time, Hans-Dietrich Genscher, assured you that "NATO will not expand to the east," and that states like Poland and Hungary could never be part of the military alliance. Because the conversion had revolved mainly around East Germany, Genscher even became more explicit, saying that: "As far as the non-expansion of NATO is concerned, this also applies in general." According to reports, you replied that you believed everything he said. So why didn't you get this commitment from NATO on paper? >> Shevardnadze: Times have changed. At the time we couldn't believe that the Warsaw Pact could be dissolved. It was beyond our realm of comprehension. None of the participating countries had doubts about the Warsaw Pact. And the three Baltic states, which are now part of NATO, were still part of the Soviet Union then. Eventually, we agreed that a united Germany could be part of NATO under certain conditions. For example, a national army limited to 370,000 members and Germany waives the right to nuclear weapons. An expansion of NATO beyond Germany's borders was out of the question. >> SPIEGEL ONLINE: At the end of March 1990, Genscher and the then US Secretary of State James Baker, talked about the fact that there was interest among "central European states" about getting into NATO. You knew nothing of this? >> Shevardnadze: This is the first I've heard of it. >> SPIEGEL ONLINE: Did you have a conversation with your colleagues in Poland, Czechoslovakia and Hungary about a possible eastward expansion of NATO in the spring of 1990? >> Shevardnadze: No, that was never discussed in my presence. >> SPIEGEL ONLINE: The German documents give the impression that Moscow counted on the dissolution of both the Warsaw Pact and NATO. Did you really think that would happen? >> Shevardnadze: That may have been discussed after I resigned from the ministry of foreign affairs in December 1990. However during my time in office it was not. >> SPIEGEL ONLINE: In May 1990, Soviet leader Mikhail Gorbachev brought up the subject of the Soviet Union joining NATO during talks with the Americans. The Americans took that very seriously. >> Shevardnadze: Gorbachev had that idea but he never took any realistic steps towards achieving this. Which is why it was never really discussed amongst the Soviet leaders. >> SPIEGEL ONLINE: Was the eastward expansion of NATO ever discussed in the inner circles of the Communist Party of the Soviet Union in 1990? >> Shevardnadze: The question never came up. >> SPIEGEL ONLINE: Did the subject play a role in the ratification process of the Two-Plus-Four agreement (where the signatories included the two Germanys and the four powers that occupied Germany after World War II) that unified Germany? >> Shevardnadze: No, there were no difficulties whatsoever with the ratification process. >> SPIEGEL ONLINE: Nevertheless, the eastward expansion happened a few years later. Did you feel, at the time, that the German diplomats deceived you? >> Shevardnadze: No. When I was the minister of foreign affairs in the Soviet Union, NATO's expansion beyond the German borders never came up for negotiation. To this day I don't see anything terrible in NATO's expansion.