Neptunes brood.

"Neptune's Brood" by Charles Stross (2013) has the problem of interstellar money, banking, and debt as the central plot. Set in set in AD 7000, money is called "bitcoin" but to deal with interstellar distances is split into three classes: "fast", "medium", "slow".

Fast money is for planetary transactions, medium money is for interplanetary, and slow money is for interstellar trade. Slow money transactions rely on a three-way crypto-verification scheme, and so trade at one-third the speed of light.

If you want a deep dive into galactic-scale banking systems and don't mind deep dives into accounting and gratuitous android sex, this novel is a good deep dive