Making amends for your sins is a recurrent idea in “Fullmetal Alchemist.” It’s what Ed and Al’s journey is about. They tried to revive their dead mother, violating the laws of nature, and paid the price (Ed lost an arm and leg, Al’s soul is trapped in a suit of armor). They’re searching for the Philosopher’s Stone to heal their bodies, but also to absolve themselves; they feel mutually guilty for what happened to the other.
At the same time, the series deconstructs the supposed nobility of self-sacrifice. When Ed nearly gives his life for Al, the younger Elric calls his big brother’s effort a waste: “Survival is the only way, Ed! Live on, learn more about alchemy! You can find a way to get our bodies back […] I won’t allow you to abandon the possibility of hope, and choose a meaningless death.”
When Hawkeye tells Ed about her and Mustang’s plans, he admonishes her, practically quoting Al’s words. Hawkeye counters that she and Roy aren’t choosing martyrdom, but duty. They can’t restore the lives they took, so they have to devote their own to helping the remaining Ishvalans. That means tearing down the system that crushes them,t and if their victims’ relatives decide they still need to be punished after that, so be it.
When Ed next sees Mustang, he refuses to return some money (520 “cens,” or coins) he borrowed earlier from the Colonel. Instead, he’ll return it when Mustang becomes Führer. Then he’ll borrow more money and keep that until Amestris is a democracy, and so on and so forth. The paltry sum isn’t the point though; it’s Ed letting Roy know that he’s joined Riza in keeping him accountable.
