using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Bill> bills = new List<Bill>();

        bills.Add(new Bill() {ActualAmount = 150, PaidAmount = null});

        bills.Add(new Bill() {ActualAmount = 150, PaidAmount = null});

        bills.Add(new Bill() {ActualAmount = 2400, PaidAmount = 2000});

        var amount = bills.Sum(x => (x.ActualAmount - (x.PaidAmount ?? 0)));

        Console.WriteLine(amount);

    }

}

​

public class Bill

{

    public double? ActualAmount {get; set;}

    public double? PaidAmount {get; set;}

}