ImmutableMoney.scala

case class Money(amount: BigDecimal, 
                 currency: Currency) {

  def plus(other: Money): Money = {
    require(currency == other.currency)
    new Money(amount = amount.add(other.amount), currency)
  }

}

MutableMoney.scala

case class Money(private var _amount: BigDecimal, 
                 private var _currency: Currency) {
  
  def amount = _amount
  def currency = _currency
  
  def plus(other: Money): Unit = {
    require(currency == other.currency)
    amount = amount.add(other.amount)
  }

}