On 19 February 1878, Otto von Bismarck spoke in the Reichstag about the crisis in the Balkans. He described the part he wanted Germany to play in settling it. He did not want Germany to den Schiedsrichter spielen, to play the arbiter, and say So soll es sein, this is how it shall be, with the power of the German Reich behind it. He saw the role as more modest, and he took an example from ordinary life. It would be mehr die eines ehrlichen Maklers, der das Geschäft wirklich zu Stande bringen will, more that of an honest broker who really wants to bring the business about. Four months later, the great powers met in Berlin under his chairmanship.
The phrase is often quoted in a smoother form: Ich will nichts mehr als der ehrliche Makler zwischen uneinigen Klienten sein, der das Geschäft zustande bringt. I want nothing more than to be the honest broker between clients who disagree, who brings the business about. This form is usually placed at the Congress of Berlin, and it has no clear source. The speech as printed in 1878 uses the wording above.