How to convert the following decimal? to String("F2")

If it's a nullable decimal, you need to get the non-nullable value first:

@item.Sales.Value.ToString("F2")

Of course, that will throw an exception if @item.Sales is actually a null value, so you'd need to check for that first.

You could create an Extension method so the main code is simpler

  public static class DecimalExtensions
  {
    public static string ToString(this decimal? data, string formatString, string nullResult = "0.00")
    {
      return data.HasValue ? data.Value.ToString(formatString) : nullResult;
    }
  }

And you can call it like this:

  decimal? value = 2.1234m;
  Console.WriteLine(value.ToString("F2"));