In direct reply to your question, make a variable that stores each player's money in the script, use this variable when you give or take money from a player. Make another variable that stores the money in their bank account. And adjust that variable accordingly when they use those commands. Finally, in your save/loading functions for the player, add these variables and you should be done. Though you may want to make a /balance command as well so they know how much is in their account.