RealGM

What happens if a player wants to sign with a foreign team while still under contract in the NBA?

Let's take a hypothetical example, Beasley dominates the league next year and puts up 25/10 and becomes anointed as one of the faces of the future of the NBA along with Lebron and Paul. Then a rich Euroleague team comes along and offers him a 8-year €120 million contact. That's equivalent to $22 mil per year at current exchange rates. Meanwhile Beasley is looking at toiling for 3 more years under the rookie scale earning ~$5 mil/year, before he can get a raise to the max of ~$15 mil/year. Furthermore, 8 years guaranteed is far more than he can get under the CBA in the NBA.

Basically can a player ignore or void his NBA contract if he doesn't intend to play in the NBA next season? In my previous example, could Beasley announce his "retirement" and then move to Europe? Or is there some legal agreement between the NBA and foreign teams to respect the contracts?

The team can sue the player to stop them:
http://nbpa.com/cba_exhibits/exhibitA.php

Therefore, it is agreed that in the event it is alleged by the Team that the Player is playing, attempting or threatening to play, or negotiating for the purpose of playing, during the term of this Contract, for any other person, firm, entity, or organization, the Team and its assignees (in addition to any other remedies that may be available to them judicially or by way of arbitration) shall have the right to obtain from any court or arbitrator having jurisdiction such equitable relief as may be appropriate, including a decree enjoining the Player from any further such breach of this Contract, and enjoining the Player from playing basketball for any other person, firm, entity, or organization during the term of this Contract. The Player agrees that this right may be enforced by the Team or the NBA.

And if they refuse to play in the last year they do not become a free agent:

A player who withholds playing services called for by a Player Contract for more than thirty (30) days after the start of the last Season covered by his Player Contract shall be deemed not to have “complet[ed] his Player Contract by rendering the playing services called for thereunder.” Accordingly, such a player shall not be a Veteran Free Agent and shall not be entitled to negotiate or sign a Player Contract with any other professional basketball team unless and until the Team for which the player last played expressly agrees otherwise.

As to international agreement I believe FIBA, Euroleague and the NBA all must honor contracts as covered by international contract and sport laws. If the NBA sued a Euro league team it would likely end up in arbitration by the CAS, the International "Court of Arbitration for Sports" based in Switzerland.
The court started as part of the IOC but became independent in the 1980s.
http://www.tas-cas.org/