What you need to ask for, is to do a "spec remix" of a song. In this case, they should be giving you the parts you ask for to do the remix, with the permission of the artist of course, and in the end, if your finished product gets big enough to make profit, you have to negotiate your pay with the label/artist, since the artist still get royalties and the label owns the license and you've added intellectual property to some1 else's and made money from it.
A lot of remixes get done both that way and just by asking to do a normal remix, whereas you'd be paying for it. Many remixers end up making more money than the original artist, b/c remixing breaks the genre barriers in some cases and allows for more prominent radio play and multimedia releases ie: vinyl, CD, compilations, etc.
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'-.SgtFoo.-'
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