My wife works for a local restarant for the past four years. She was making six dollars per hour working the bar and waiting tables.

As a general matter, employment relationships in the United States are "at will," which means that either party may insist on changes at any time for any reason or no reason at all. The possibility of a retroactive change is problematic, because she had apparently already completed the work for which she was then paid less than the agreed amount. I'm guessing that not much money is involved in the retroactive adjustment, so that may not be worth pursuing. The prospective adjustment is lawful and your wife's remedy is to quit.