Montgomery's promised successor and civilian appointment authority

A recalled exchange has Montgomery announcing that he has chosen his successor as Chief of the Imperial General Staff. The reply, attributed tentatively to Anthony Eden, is that the civilian authority has also made that choice. When Montgomery says he has already promised the appointment, the response is to withdraw the promise. The anecdote illustrates the limit on a commander's ability to commit an appointment that another authority controls. It appears as an analogy for developers who had sold plots before resolving the estate's access problem.