It is complicated a bit at the beginning. In all the sports I know from US, they have something like salary cap, it exists also in MLS but it's just not called like that.
Basically, you got normal transfer budget and salary budget, but you will not be allowed to use all of it. Let say you got 15mil in transfer budget, but you're allowed to use only 3,5mil throughout a season.
Salary budget is also limited to some number per season, but there are some rules that gives you an exceptions - that's why clubs can afford to have people like Zlatan there. It's called a Star player contract. You can have maximum 3 players like this in your club. Their salary is not counting towards your Salary Cap limit as a whole, so no matter that you need to pay Zlatan 450k - it will take only 8k from your salary limit

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Additionally, you can swap players between MLS teams without really paying for it, you simply swapping player for another player and/or plus some other options, like for example place in draft, extra foreigner space in team or extra money towards your limited transfer budget.
Not sure if my english is enough to perfectly explain all of it.
Anyway - IT IS interesting and you need to use transfers with some strategy, you can't just buy players as you want with all money you've got there. Thanks to it league is also more equal.
Might sound a bit complicated, but trust me, it's not that bad and hard to get it

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