Brown said the contract offer from San Francisco was an insult.
"[I]t wasn't what I wanted and it was to me more of a slap in the face," Brown told NBC Sports Network's ProFootballTalk. "But I understand that it's a business and I understand that they have to do what is best for them and it's no hard feelings.
"But what they offered me definitely wasn't what I wanted and definitely wasn't worth my value of what I've done in this league and how well I played these past 3-4 years in the league. So it's understandable and like I said there's never any hard feelings. They are doing a great job over there and I wish them the best."
The contract Brown signed with the Raiders reportedly was a fully guaranteed one-year deal at $3.5 million. It separately had been reported the 49ers were willing to give Brown a three-year, $10 million contract.
The 29-year-old corner has started 42 games the past three seasons with 39 passes defensed and six interceptions.
It's clear that Brown is hoping to become the next Brent Grimes. After signing a "prove-it" contract with a clear opportunity to start in Oakland, Brown hopes to cash in next year when he is 30.