The team announced that they don't expect to have Dilly Duka back for the 2016 MLS season as he explores his options in Europe. The Impact do, however, retain his MLS rights.
When Zarek Valentin decided to return to MLS earlier this month, the Montreal Impact acquired an international roster slot from Portland in exchange for him. Valentin had been playing in Europe, but IMFC retained his MLS rights. Thus, the Impact were compensated for his services.
It appears that a similar situation has come of midfielder Dilly Duka. Although out of a contract with the team, Duka's MLS rights are still held by Montreal (perhaps similar to a "restricted free agent" if this were the NHL).