He may not own the copyright, but if he has a contractual royalty agreement then he certainly retains legal claims on use of the copyright.

Whomever owns the copyright is entitled to do whatever their control of it allows them to do, but they are obligated to fulfill any claims others have on the revenue derived from those uses.

Do you have an illustrative case in mind? The claim is it's a slam dunk, I'm trying to understand the basis of the claim.

Not exactly a case, but an example: it's the same reason why MS Windows will have, on "cover", "copyright Microsoft", not a complete list of licensed components for some of which they might still be paying royalties. Similarly in other works.

In this case, the book as a whole would be copyright Star Wars Corporation, probably a holding entity specially to handle multiple involved copyrights (the copyright to Star Wars itself, etc.), and unlike a Windows install media you won't find a page listing "this work contains parts copyright X, Y, Z". But the contractual obligations still exist.

Both the previous reply and this sound more like convoluted motivated reasoning towards the desired outcome - that the author gets paid. I'd love to see the author get paid too! But I don't think these are particularly strong analyses of what looks like a complicated contract case. As far as I can tell, nobody (in thread) had as much as checked the copyright ownership. But I'd love to be wrong and learn me a thing or two!

While I'm not privy to the specific contracts in question, who holds the copyright probably isn't relevant to the dispute, at least not in the most obvious way -- namely, it doesn't matter that the copyright was never held by Foster. The relevant questions are, I suspect, (a) whether or not the contract included royalties to be paid to Foster, and (b) whether the obligation to pay said royalties survives transfer of ownership of the copyright.

A lot of "work for hire" contracts don't pay royalties, just a flat fee, but I don't think we'd be talking about this at all if this were the case here -- Foster was evidently being paid royalties until Disney took over Lucasfilm. So the dispute is over the second question. Disney is implicitly arguing that they get out of paying royalties to every single author who created works for companies that Disney subsequently bought, regardless of whether those contracts specified royalties that the predecessor corporations would have otherwise been paying.

How is being required to fulfill their half of the contract complicated?

Why would a Wookiee, an 8-foot-tall Wookiee, want to live on Endor, with a bunch of 2-foot-tall Ewoks? Something morally trivial can be legally complicated.

Disney's contractual obligations to pay the originally-agreed royalty aren't copyright-dependent.

On top of that, these kind of contracts often come with a 'flagrant breach of contract means that copyright reverts to the original author' type of clause.