using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<string> allDates = new List<string> { "2021-01-02", "2021-01-03", "2021-01-04" };

        List<Balance> balances = new List<Balance>

        {

            new Balance { Id = 1, Amount = 233, Date = new DateTime (2021, 01, 02) },

            new Balance { Id = 2, Amount = 442, Date = new DateTime (2021, 01, 03) }

        };

        var lookup = balances.ToLookup(x => x.Date.ToString("yyyy-MM-dd"));

        var result = (from a in allDates

                     select new 

                      { 

                          Date = a, 

                          Amount = lookup[a] != null && lookup[a].Count() > 0 ?  lookup[a].First().Amount : 0

                      }

                     ).ToList();

        foreach (var item in result)

        {

            Console.WriteLine(String.Format("Date: {0}, Amount: {1}", item.Date, item.Amount));

        }

    }

}

​

public class Balance 

{

    public int Id {get;set;}

    public decimal Amount {get;set;}

    public DateTime Date {get;set;}

}