I'm not saying you have to negotiate a done deal, but players, teams and agents talk to each other. Even if they don't have a deal in place before executing a trade (which moon pointed out is rare) that doesn't mean you don't either have A. a frame work for one, or B. an ideal of what they're looking for.
E.G. Maybe Skubal is looking for 400 million. which has been floated, so now you know about what it's going to take, maybe he takes a little under 400 if you're offering higher AAV....or maybe not. But you have a ballpark figure and if you're not comfortable with that you don't make the trade.
I get this is fantasy, lots of moving parts, but so is pretty much everything we talk about until it happens.