Span has been battling back spasms since June 7. The move was retroactive to Tuesday.
Barrett missed 24 games with a right bicep strain after going 3-2 with a 5.06 ERA in 30 appearances. He threw three scoreless innings over four rehab outings with Class A Potomac.
Difo, 23, rejoins the Nats for the third time this season. He has appeared in six games, going 1-for-6 at the plate.
Cash Kruth is a reporter for MLB.com. Follow him on Twitter at @cashkruth. This story was not subject to the approval of Major League Baseball or its clubs.