You can view several salary-related data points by going to the My Team panel and selecting "Salaries." Across the top of the screen you'll see the current year's salary cap, the amount you're currently spending on salaries, your "cap space" (how much more money you have to spend), and the rookie reserve (which really only comes into play after The Draft).
More importantly, you get to see your cap penalty for both the current year and the next year. During Contract Negotiations, players will demand both salary and a signing bonus. Signing bonuses are guaranteed money, and if you release a player with a signing bonus, the amount of money you would be paying him for his bonus is added to your cap penalty, thus effectively lowering your specific salary cap. It's usually a net positive to release a player to free up some money from your salary cap, but releasing too many players in the same season will seriously hamper you in future years. Sometimes it makes more financial sense to keep a veteran languishing on your bench than throwing millions of dollars down the drain by releasing him.
Free agents signed during the season and late-round picks from The Draft often have no bonus money and can be released without cap penalties. If players are traded rather than released, your team will not suffer a cap penalty, so you may want to look to trade a player for even a low-round draft pick if you need to unload a particularly heavy contract from a player you deem unworthy of it. However, note that even the AI is smart enough to look at contracts and they may be unwilling to give anything up (even as low as their 7th round draft pick) for a grossly inflated contract.
The Salary Cap increases every year, and the Madden NFL engine always favors real-life players over generated fictional ones. This means the longer you play Madden NFL, ironically the less important the salary cap becomes because player contracts will not inflate at the same rate as salary cap increases. Players on the injured reserve list will have their salary cap returned without penalty, though you won't get their bonus money refunded. You can put a high-value player on the IR even if their injury doesn't necessarily warrant it if you want to free up their salary, though be warned the player will usually be unavailable for the rest of the season.
Beyond that, there are ways to manipulate the engine to stay under the cap but ensure you have good players. The most important thing to understand is the engine itself: in Madden NFL, certain attributes (based on position) are often more important than others, and your personal play style should be considered above all. For example, quarterbacks are highly rated (and thus demand huge salaries) if they have high throwing power and throwing accuracy. But if you prefer a West Coast offense and rarely throw deep, you won't really need that expensive 91-rated quarterback. He'd be nice, sure, but it may be an unnecessary expense you'll want to jettison later.
This is especially true based on your skill and league settings as well. If you're playing in a league without injuries, you can spend basically no money on your bench and save money that way. Or, if you're especially good at running with the ball, you may not need a highly rated HB since a mid-tier one could serve you just fine.
In this way, you can exploit the AI's preferences. The AI will always try to field a good, full team and does not consider league settings, so even with Injuries off it will try to sign 53 players. (This is because simulated games and played games take place in two different engines, but that is beyond the scope of this wiki.) So while the AI needs to field a full team, you would only need the barest minimum of players, and therefore can spend more money for better but fewer players.
Further, players are coded to greatly prefer a signing bonus over their salary. If you're playing Be a Coach where bonus money is unlimited, you can get away with signing contracts with large amounts of bonus and tiny amounts of salary. While this will be severely damaging if you later want to release the player, in the short-term it will help you stay under the cap. Just be sure to only give that treatment to the stars who you know you won't want to release.
Assuming you're playing fair in Be an Owner with full injuries and the salary cap active, you can still keep your costs down by over-investing in your starters and under-investing in your bench. Even real-life teams tend to have a few superstars, many good (mid-80s) players in other positions to support them, and the rest filled out with whoever they can get their hands on. Just keep your Scheme and personal play style in mind, and it's unlikely you'll have too many tough decisions on balancing the budget.
