Step 4: Transfers are all-or-nothing

import java.util.*;

public class Main {
  static class InsufficientFundsException extends RuntimeException {
    InsufficientFundsException(String m) { super(m); }
  }

  static class Account {
    final String name;
    long cents;
    Account(String name, long cents) { this.name = name; this.cents = cents; }
    void withdraw(long amount) {
      if (amount > cents) throw new InsufficientFundsException(name + " cannot pay " + amount);
      cents -= amount;
    }
    void deposit(long amount) { cents += amount; }
    public String toString() { return name + "=" + cents; }
  }

  // All or nothing: if the deposit side fails, the withdrawal is undone.
  static void transfer(Account from, Account to, long amount) {
    if (from == to) throw new IllegalArgumentException("cannot transfer to the same account");
    long fee = amount >= 10000 ? 100 : 0;                  // 1.00 fee for large transfers
    from.withdraw(amount + fee);
    try {
      if (to.name.equals("frozen")) throw new IllegalStateException("account frozen");
      to.deposit(amount);
    } catch (RuntimeException e) {
      from.deposit(amount + fee);                          // roll back
      throw e;
    }
  }

  public static void main(String[] args) {
    Account ada = new Account("Ada", 50000);
    Account sam = new Account("Sam", 1000);
    Account frozen = new Account("frozen", 0);

    Object[][] requests = { { ada, sam, 2000L }, { sam, ada, 99999L }, { ada, frozen, 500L }, { ada, sam, 10000L }, { ada, ada, 1L } };
    for (Object[] r : requests) {
      Account from = (Account) r[0], to = (Account) r[1];
      long amount = (Long) r[2];
      try {
        transfer(from, to, amount);
        System.out.println("OK    " + from.name + " -> " + to.name + " " + amount);
      } catch (RuntimeException e) {
        System.out.println("ERROR " + from.name + " -> " + to.name + " " + amount + ": " + e.getMessage());
      }
    }
    System.out.println(ada + " " + sam + " " + frozen);
    System.out.println("total " + (ada.cents + sam.cents + frozen.cents));
  }
}

OK    Ada -> Sam 2000
ERROR Sam -> Ada 99999: Sam cannot pay 100099
ERROR Ada -> frozen 500: account frozen
OK    Ada -> Sam 10000
ERROR Ada -> Ada 1: cannot transfer to the same account
Ada=37900 Sam=13000 frozen=0
total 50900