Sancho is now in his third season with BVB.
Former City defender Richards said: “Sancho has been confident enough to take that leap so early. I don't think my parents would have allowed me had I had that many options at that age.
“But I didn't need to leave City because I was playing in the first team at 17, so the scenarios were a little bit different, but fair play to Sancho.
“He wanted to play at City but with the players in front of him he was never really going to get in. He took the plunge and now he's worth £100m minimum now, so I've got a lot of respect for him and credit to him and his agent."