using System;

public class Program

{

    public static void Main()

    {

        var json = "{ \"totalIncomeBeforeTax\": \"1234\" }";

​

        Console.WriteLine("Json = {0}", json);

        var obj = System.Text.Json.JsonSerializer.Deserialize<Poco>(json);

        Console.WriteLine("TotalIncomeBeforeTax = {0}", obj.totalIncomeBeforeTax);

    }

    private class Poco {

        public decimal? totalIncomeBeforeTax{get;set;}

    }

}