Don’t have all the details myself but important to remember actors are paid for more than what they do on set. There is still plenty of prep work as well as promotional material and rights for using their image for advertising and merchandising. Even if his screen time was drastically reduced there probably was still plenty of demands for media appearances and other obligations in the contract that would consume as much if not more time and energy (which equally restricts or prevents the actor from seeking other competitive jobs while dedicated to one long term project.)

So while there may have been to grounds to reduce the offer of pay somewhat, cutting by over half while still largely expecting the same work role even when considering reduced screen time possibilities, it’s entirely justified for the actor to refuse the role.