We really need more of a description than that... better yet a photo.
As a warning, however, I will say never to buy a fish until you've ID'd it and know about it. Many fish are not compatible with inverts. Some are difficult to feed, some ned more room than your tank, etc.
If he is attacking the shrimp you can either let him eat the shrimp or take him back. Don't think there are any other options unless you have another tank set up.