Billings and Stephen Strasburg were teammates at San Diego State University in 2007.
To clear room, the Nationals outrighted Ryan Mattheus and Pedro Florimon was claimed off waivers by the Pirates.
Brian Goodwin, A.J. Cole, Wilmer Difo, Matt Grace and Matt Skole are among the players eligible for Rule 5 draft protection.
The catcher, known as the Onion, was released by the Nationals to clear a 40-man roster spot.
The former Class AAA Syracuse manager, Nationals third base coach and minor league instructor was hired by the Rangers.
The left-handed reliever has blossomed into a prospect and had his finest season to date, reaching Class AAA Syracuse.
The Nationals keep the recently-released former prospect in the organization.
The Nats brought back D.C. Native Emmanuel Burriss and Manny Delcarmen.
The Nationals added Eric Fornataro off waivers from the St. Louis Cardinals.
The second baseman is hitting .205 in 12 games, but is known for his high energy and work ethic.