I need some advice. I recently found out that some of people that I have more experience than and at times train, unjustifiably make more then me. I brought it to my manager's attention and demanded more money. Now, one month later, my manager got the required approval to give me an increase, but is trying to get me to commit to staying with the company for at least another 6 months. My manager does not want to run the risk of me leaving right after I get the increase. Can my manager withhold the increase if I do not commit? What are my options?

Yes he can withhold your increase for almost any reason. He was given permission to increase but not a mandate. You wanted the money, he is offering it so just say you will stay. Unless you sign a written agreement, don't do it, you are not legally bound to stay. It is silly on his part since if you leave after the raise you won't be taking much new cash with you, you will likely save them money.

They can do pretty much whatever they want especially if you live in a 'at will' state
At-will employment is a doctrine of American law that defines an employment relationship in which either party can break the relationship with no liability, provided there was no express contract for a definite term governing the employment relationship and that the employer does not belong to a collective bargain (i.e. a union). Under this legal doctrine:

“ any hiring is presumed to be "at will"; that is, the employer is free to discharge individuals "for good cause, or bad cause, or no cause at all," and the employee is equally free to quit, strike, or otherwise cease work.[1]

Short of discrimination, federal law and company policy they can do anything.

Federal policy would be they are required to pay at least minimum wage.

Thanks!