transfer.rb

class Account
  ...
end

class Transfer
  def self.exec(acct1, acct2, amt)
    Transaction.begin do
      acct1.balance -= amt
      acct2.balance += amt
      acct1.save!
      acct2.save!
    end
  end 
end

Transfer.exec(Account.find(1), Account.find(2), 500.00)