The move frees up $6 million in salary cap space heading into free agency.
The 29-year-old Langford had been entering the final season of a four-year, $22 million free agent deal. He's a former third-round draft pick by Miami.
Langford started every game in 2012 and 2013 and had a career-best five sacks in '13. He made four starts last year and had one sack.
Donald was the NFL Defensive Rookie of the Year and had nine sacks and two forced fumbles.
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