While not fantasy, I remember something cropping up in the monday:history threads about banking and usury.
You might want to hit google and do a search for medieval usury. IIRC, the are laws regarding banking back to the Romans. The notion of compound interest is nothing new and a similar "common sense" set of banking laws seems to exist in all established cultures. I use some simple bits of banking in my campaign to help tie the characters into the world.
If you have MMS:WE check the section on the patriciate as it gives a reasonable explanation for adventurers as (semi-willing) bankers.