Symbolism, Imagery, Allegory
As the great Joel Grey and Liza Minnelli once sang, money makes the world go around—and boy, is that a hard reality for Barbara Undershaft to swallow.
Barbara fancies that the Salvation Army is divorced from the violence and greed in the world, devoted only to serving the poor and saving their souls. However, when the Army gets a gigantic donation courtesy of her father and a whisky magnate named Bolger, the organization jumps on the money without a second thought—after all, they'd be in danger of shutting down without it.
Even though Barbara is initially horrified that the Army could be "bought" like that, she ultimately seems to resign herself to the fact that this is the way the world works; without money and power, you aren't going to get anything done—and then the "bad guys" win.
So what exactly does money symbolize or represent? Well, it seems tied to power and autonomy, if we're looking at the Salvation Army donations as an example. Even though on one hand the Army is becoming beholden to certain interests (i.e., the arms and alcohol industries) by accepting the donations, they're also getting what they need to continue and further their mission.
Undershaft himself suggests money's larger symbolic weight when he envisions Barbara taking up his company's "gospel":
Yes, money and gunpowder; freedom and power; command of life and command of death. (2.273)
If the items in this list are to be interpreted as parallels, then he's definitely associating money with larger ideas such as freedom and "command of life."
Also, let's not forget the repeated references to the "sovereign" that Bill initially offers as a donation to the Army. Sure, it's just a currency denomination, but the word also refers to a leader who is powerful and autonomous enough to rule the roost. Perhaps that coin is symbolic of the way money makes you "sovereign," too.
In any case, money definitely makes Major Barbara's world go around, conferring power and independence on those who accept it . . .