mob
player
var
yen
Organization/organization
verb
deposit(var/amount as num)
if(yen >= amount)
yen -= amount
organization.bankAccount.addYen(amount)
Because you didn't make the organization var tmp, it will save with the mob. That's bad news because it will save an outdated copy of the organization. Whenever you're working with shared atoms or datums like this, it's crucial to be aware of what might accidentally save along with your mob.
What you should do instead is make that /tmp and check, on load, which organization that player belongs to.
Note that O.bankAccount is the /Account. It has a var called yen, and a proc called addYen(). You cannot call O.bankAccount.yen.addYen because O.bankAccount.yen is a var, not another object.