That's true. With that said, while the Cubs didn't have to trade a salary, they have, with the Tucker trade, backed themselves into a corner needing to trade an OF'er right now. It's seemed recently that the Cubs and Yankees were close, and that very well may be the case. But we've also seen a few things that make you wonder if there's some small snag.
A situation of trading for Castillo would obviously represent a different case in that it'd be assumed most of the year the Cubs would have to move someone out. But I think the Yankees, as well as other teams, know the Cubs need to move an OF'er right now, too.
In much the same way that the Cubs probably saw Tucker as a thing they had to do now (and I don't blame them one bit - good trade!) and worry about the Bellinger part later they may seem a trade like Castillo too good to pass up (I'm not sure I'd agree with them, I'm just using it as an example) now that they'd worry about the 2nd step later.