Being valued, winning shit, feeling like a manager is going to grow your game, being part of a project, competition, could all be just as important to some players as maximizing your salary. Facilities, manager support, playing with your friends, also weigh in. Additionally, if you're a key cog, or the main guy to where you go, you can always get a new contract when you show that you're that important (which a lot of clubs, especially Liverpool are happy to reward performing players) to warrant your contribution.
We also aren't talking about a guy at the end of the line who will never get another deal, or another move, or what have you. If you take less now, but turn around and get more accolades, more stability, more achievements individually, and as a club, you likely will get more money accordingly. That's also to say, If I go from 80k a week to 170k a week, that's a hell of a raise. Especially if there's escalators or performance based incentives. It's not 250k a week, but I'm not on the street either.
If I'm Bobby Firmino, the stability of Liverpool is nice, but it's likely my last deal. I need to get paid what I can. If I'm Mason Mount, and I believe in myself, I could afford to take a bit less now, for more reward personally and potentially financially down the road.
For some players, and some agents, the dollars are all that matter. But I don't really know what's at play here. There's clearly some element of concern about the project, as he's likely getting offered a pretty good package to stay.
