Aldo will not be able to breach his contract, I don't care how good his lawyer is. The UFC has been doing this for awhile and plenty of fighters have tried in the past without success.
The UFC has been working hard at penetrating the Brazilian market and will not let a top 3 fighter go under any circumstances. They can talk all the shit they want, in the end, if Aldo tries to fight for another organization, the UFC will sue him.
What I find amusing is that if Aldo was more of a draw, he would have a great deal more negotiating power. Look at Cyborg; I am not a big fan of hers but she has clearly made an effort to learn passable english for interviews, and has actually become somewhat marketable.
Aldo has the personality of paper mache, barely speaks english, has pulled out of multiple fights over the years. He headlines a card and unless he is fighting Conor, it will be a below average PPV and gate (unless held in Brazil).
His options here are to retire (which he won't, someone told him this bluff will work), Suck it up and fight who his employers tell him to (he does get paid for this....), or kick a ref in the ass.