Okay, this guy is pretty much just a straight-up comic figure. He is as stereotypical a stereotype as this set of stories ever produces. He spins Tevye a story about basically using magic to turn his hundred rubles into eight hundred, and, if you believe that—well, we've got this really good investment opportunity on the Brooklyn Bridge for you.
Of course it doesn't take Tevye long to figure out that he's "a fly-by-night, a who knows what, a wheeler-dealer, a manipulator" (3.2). And it's too bad for Tevye, but… yeah, it's also kind of funny. As Tevye keeps talking, his stories start to move away from this funny slice of life premise, and instead take up larger and more serious themes. But this? Feel free to laugh.