If you have a contract with them, that would dictate what you/who your working with can do. This is why you see
"_______ appears courtesy of _______(insert label)"
From what i get, this isn't the issue. First, if you artist respects your label, they would be releasing their album, and working hard to do so, under your label. Therefore, it wouldn't be very intelligent to leave you in the dark here. Secondly, if he plans to work with other people on his album, although you should have spoke about that with him/her before asking us, or signing him for that matter, it's not necessarily bad.
I would have to ask a few questions before giving any advice. Are you paying the artist in question? Are you paying for a song when done, or by profit of said album? Have you invested money in his album and gave him any sort of budget to work within? If the answer is no to all these then he has every right to be doing whatever he wants.
I really hope you arn't the beat maker who becomes the producers, then the label executive with retaining artists as your main goal, but not compensating them/making it worth there while to stay. Then feel like they have an obligation when you don't benifit them. I don't want to jump to this conclusion though, just bringing it up.
