The deal is contingent on whether Barnes makes Detroit's 53-man roster.
"Here's a football team that needs a corner," Washington coach Mike Shanahan said. "Here's a third-round draft pick that's got some great speed. If he makes the team, we get something in return and they get a good football player."
The Redskins were preparing to cut Barnes had they not found a trading partner. He was behind starters DeAngelo Hall and Josh Wilson on the depth chart and was a contender for playing time as a nickel back during training camp.
The 2009 third-round selection played in 29 games with three starts over three seasons with Washington. He has three career interceptions.