Well, at least he didn't self-leak it.
However this has turned out to be a big lie by Comey as his own memo (dated January 7, 2017) of a private conversation he had with then President Elect Trump on January 6, 2017 at Trump Tower shows Comey wrote he saw Trump laugh at Comey’s mention of Russian prostitutes in the now infamous Russian Dossier.
So either Trump laughs at it, or takes it too seriously, he can't win for the psycho-analysts.
Personally, I found the dossier claims with so-called "Russian prostitutes" rather funny too.