public class BankAccount

{

  public long Number {get; private set; }

  public decimal Amount { get; set; }

}

public class PaymentGate

{

  public void Transfer(BankAccount @from, BankAccount to, decimal amount)

  {

    if(@from.Amount >= amount)

    {

      @from.Amount -= amount;

      to.Amount += amount;

    }

    else

    {

      throw new InvalidOperationException(string.Format("Insufficient money left on account: {0}", @from.Number));

    }

  }

}