account.clj

(defn transfer [from-account to-account amount]
 (dosync
  (if (> amount @(:balance from-account)) 
   (throw (new Exception "Not enough money!")))
  (alter (:balance from-account) - amount)
  (alter (:balance to-account) + amount)))

(defrecord Account [account-no balance])

(def account-a (Account. 1 (ref 1000)))
(def account-b (Account. 2 (ref 800)))

(transfer account-a account-b 300)

@(:balance account-a)
=> 700