Here are some big picture details from The Hollywood Reporter:
So if these movies -- however many he makes as Gambit -- make serious coin, Chan needs to be shown the money. But it is a bit awkward now, considering how many fans were fine -- even happy! -- about the idea of Channing dropping out, because they don't see him as a good Gambit. Then again, it's rare for fans to be 100 percent supportive of casting, especially comic book casting. Channing is a charmer and he'll probably win everyone over, we just have to wait and see.
One thing that needed figuring out was the extent of Tatum's involvement in the X-Men universe. Fox sees Tatum's Gambit, a mutant who harnesses kinetic energy, as one of the anchor characters of the X-universe, akin to Hugh Jackman's Wolverine, someone who would anchor his own movies and appear in numerous others. It's a commitment that could theoretically last more than a decade. (Jackman has been portraying Wolverine since 2000, for instance.)
Sources say that backend compensation was also a sticking point.
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