Unfortunately this is very common when you're dependent on employer for Visa - I was once in his position myself... even found out that I was the lowest paid engineer in the team. My direct manager raised hell and I got a little raise but the damage was done. I moved on as soon as I got my green card... and founded my own company, never to be employed again.
I was also in this situation 25 years ago. I was 25% under the market. During the green card process, the visa official reviewing my application made the comment and they raised my salary! That's the world - people will screw you if they can.
