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Maybe Neymar didn't get enough money like Mbappe in PSG but the fact about Neymar is he was not a bold player like Mbappe and it was normal to see him getting less salary however as you already said if he wants to get more money in the next season he can join the Saudi teams but Neymar is still in the form in which he can shine in Europa for more years.
I think its not only about money that Neymar thinks about a career that has to reach to the top in a competition like UCL. I wont compare Mbappe with Neymar but its really very different in terms of performance because Neymar after suffering an injury his performance dropped and he could not perform as strong as he used to and maybe if he did not get injured maybe after Mbappe left PSG and moved to Bayern Munich, of course Neymar ready to replace his position and his salary will rise.
this is about loyalty to the team with the goal of achieving that must be obtained.
Im also still unsure whether Mbappe wants to join one of the teams in the Bundesliga because Mbappe himself needs a team that can take him to the UCL final and IMO, not Bayern Munich. but lets see where Mbappe will decide because we here dont know for sure where he will go because there is no confirmation from Mbappe.