Short version first, if you need more help, I'll be happy to oblige.
If Camavinga is a 'nice boy' and would like Rennes to receive some recompense and/or isn't totally averse to the idea or even vacillates between wanting it but also wanting his preferred clubs (Devil: f*ck 'em, f*ck 'em all in January, Angel: no, Eduardo, noooo think of Rennes, think of what they did for you! Camavinga: Shut up, both of you!) then Rennes have some leverage. Depending on how 'nice' Camavinga is - AKA how much Camavinga really doesn't like the idea of Rennes getting zilch - they could have a considerable bit. It's that simple.
Whether or not they choose to apply said leverage depends on the personalities of the executives/administrators in question: 'We know Eduardo is a conscientious person, we have a choice to make; put light pressure on him or not'. Them publicly saying 'one thing is for sure, he will not go for free' could be that indirect pressure.
If he's had conversations with Rennes where they've even discussed this stuff outright, then it gets even more complicated.