Basically, the idea of a bank is subtract the money from your inventory, and add it to a variable (yea, you need to check it against how much you've got). To withdraw, remove from variable and add to inventory.
To interest, do a time system and for every 7 days, add 10% or something.
Finally, if you have equal/more than the variable limit (I think it was 999,999) remove that from the bank, and add 1 Million dollars (as either an item or a variable).
If you wanna do an foreign currency, instead of using gold, just add to a variable, then have some kind of custom menu that shows the currency types. Then do an exchange, like 5 gold is 3 adamant. Add and subtract, same as banking.