C-Bills could simply be the reference money, the only "stable" or accepted money for exchanges.
From what I remember, I think it is backed by ComStar thanks to its communication monopoly.
As for the other powers, they are likely to back their money by their own economical power and their capacity to attract foreign money.
However, this means that money can flow through the Inner Sphere, which seems rather unlikely (otherwise, the Capellan Confederation would be in dire straits).
This makes me think that the C-Bills is used as the reference money for trades and exchanges between the different States, which means that ComStar might also act as the Inner Sphere Central Bank.
Now, the real question would be how do the rates change over time?
Perhaps, ComStar could also act as a notation agency, issuing ratings over the different States and their bonds or other loans.
The real question is who buy these bonds and loans?