I would suggest that you consider offering crediting the musician/vocalist - for someone wanting to make a name for themselves that might be a reason that they might consider taking less money for their performance.  Bands have done that for eons.
 
It is also definitely a good idea to get it all worked out prior to the session(s).
 
Bob Bone

robert_e_bone
I would suggest that you consider offering crediting the musician/vocalist - for someone wanting to make a name for themselves that might be a reason that they might consider taking less money for their performance.  Bands have done that for eons.
 

I do that anyway - just thought it was honest.  I don't want someone thinking that I'm saying I can play the piano as well as what's on the record.  It wasn't me, it was John.
And I've always been credited when I've played on someone else's project.  Even if it was just a tiny guitar lick, they've added my name to the album credits.  I thought that was the norm.