Of course not. You're getting a good deal for your money, and somebody who is down on his luck is getting a leg up and an opportunity to advance his situation. Believe me, that migrant laborer wants the job as badly as you want to give it to him.
It would only be immoral if you forced him to work for you, or if he forced you to hire him, for wages other than those you would otherwise be mutually comfortable with. If you're worried that you'd be exploiting him, you can always pay him more, or do as Vegaspilot said and throw in a six-pack! ;-)