Poor Mr. Riah is the only non-Christian man in this book. And in the world of 19th-century Britain, non-Christians (and especially Jewish people) were almost considered a separate species. Mr. Fledgeby uses Mr. Riah's Jewish identity to his advantage when he traps Mr. Riah into a work contract and forces him to be the face of his (Fledgeby's) money-lending business.
He knows how willing people are to think of Riah as a greedy Jewish stereotype, so he uses this social prejudice to his advantage by making Mr. Riah the scapegoat for all his shady dealings. As Fledgeby says at one point,
He has got a bad name as an old Jew, and he is paid for the use of it, and I'll have my money's worth of him. (14.13.18)
The more we get to know Mr. Riah, the sadder it is to see him exploited so much by Fledgeby. He's a nice guy. Worse yet, Riah eventually realizes that by playing Fledgeby's game, he's giving a bad name to Jewish people everywhere. As he says to Jenny Wren,
I reflected—clearly reflected for the first time, that in bending my neck to the yoke I was willing to wear, I bent the unwilling necks of the whole Jewish people. (18.9.15)
In the end, though, he finishes his work contract with Fledgeby and the whole world finds out who the real villain is. But there's something deeper here, too. Dickens uses Riah's story to help all of us realize just how irrational we're being when we become prejudiced toward entire races or religions… and just how easily we can be manipulated into such prejudices.