In exchange, the Galaxy received the No. 3 position in the MLS allocation ranking.
The allocation ranking is used to determine which MLS club has first priority to acquire a U.S. national team player who signs with the MLS after playing abroad or a former MLS player who returns to the league after having gone to a club abroad for a transfer fee.
Opare, 23, was in his second season with the Galaxy after being selected in the second round of the 2013 draft out of the University of Michigan.
He appeared in 12 games with the Galaxy, including six this season.
"This is a move that gives us the necessary flexibility to strengthen our roster," Galaxy Coach Bruce Arena said in a statement. "We thank Kofi for his contributions here and wish him the best in D.C."