I would think it is entirely legal. You signed a contract for a year so you could get a visa for one month, but the visa (as is usually the case) was issued for a year based on the contract having been signed for a year. The employer controls the release letter as he has your signature on the contract agreeing to work for him for a year. Regardless of what you may have told him verbally, he's holding the signed contract.

My guess is the only real option you have is to hopefully work out with him some sort of monetary compensation for release from the contract.