Major League Soccer’s 2015-2020 collective bargaining agreement gives MLS players the option of becoming free agents for the first time in the league’s history.  Players who meet the following requirements for free agency are able to negotiate their next contract under certain restraints imposed by the league in the CBA. “Free Agency for out-of-contract players is available for Players who will be at least twenty-eight (28) years old in the year in which the immediately preceding League Season concluded and have at least eight (8) MLS Service Years.”  Furthermore, it’s important to understand the CBA’s Out-Of-contract Player Eligibility:
"A Player who meets the eligibility requirements and who will be out-of-contract may select his Team (from a list of interested Teams) pursuant to the following parameters, and in accordance with Section 29.9:
(i) Player shall be included on a list of Players eligible for Free Agency distributed by MLS to all Teams and the Union.
(ii) Player may re-sign with his current Team at any compensation subject to League approval in its discretion at any time prior to the beginning of Stage Two of the Re-Entry Draft. After that, a Player may re-sign with his current Team, but only at the compensation allowed under Section 29.6(b)(iii), unless otherwise agreed by the League.
(iii) Player may be compensated at the following base salary above his previous year’s base salary:
Players earning up to $100,000 in base salary: 125%;
Players earning between $100,000 and $200,000 in base salary: 120%;
Players earning more than $200,000 in base salary: 115%; or
Such greater percentage determined under Section 29.8 "
So, players have greater power over their contracts and destinations throughout the league than they did before 2015. Though, there is a kicker; MLS operates as a single-entity. MacMillan highlights an important point, “The deal allows free movement, but it doesn’t let different divisions of the single-entity artificially bid up the price in the absence of independent evidence of market value via an offer from outside of the single-entity” (Marquette Sports Law Review, 507-508). So, while the league does have a free agency period, the league still possesses significant power over its teams and players.
Obviously, there is no legal issue at hand being that the CBA was negotiated and agreed upon by both parties (MLS & MLS Players Union) but it's important to note that because of the league’s unique corporate structure, its free agency period doesn’t grant MLS players the power that professional athletes have in other professional sports leagues.