I dont think Kohl was the one who needed convincing here, but Genscher certainly played a positive role during the negotiations for German reunification, and in Germany's foreign policy in general.
Perhaps, but Kohl relied naturally on Genscher's sagacity and foreign policy negotiating skills! If Genscher's gamble had failed, his "boss" would probably never have forgiven him! As you recall, when Reagan uttered his now famous sentence "Mr. Gorbatschow, tear down this wall!" - Herr Gorbatschow, reissen Sie diese Mauer ab!, he had little encouragement from his Secretary of State. The US was very cool concerning detente or reproachment politics with either Moscow or Berlin.