So, as I understand it, the restaurant is taking the transaction fee out of your tip? Well, first let me say, that those of us with businesses understand that credit card transaction fees are part of our overhead. However, as a server you are not self-employed, you are an employee. They didn't tell you in advance, so it is really a breach of contract.
You may want to consider calling the labor authority in your state and seeing what they think. They may have broken state labor laws. Usually, any type of wage issue will get someone out to investigate.