Strange entity updating in Entity Framework Code-First

I'm solving the problem with updating entity before saving to database and got strange behavior.

I'm using Entity Framework 4.1 Code-First in ASP.NET MVC 3 web application. Here is model:

public class Order
{
    public int OrderId { get; set; }
    public int CarId { get; set; }
    public DateTime BeginRentDate { get; set; }
    public DateTime EndRentDate { get; set; }
    public decimal RentPrice { get; set; }
    public virtual Car Car { get; set; }
}

public class Car
{
    public int CarId { get; set; }
    public string Brand { get; set; }
    public string Model { get; set; }
    public string NumberPlate { get; set; }
    public decimal RentPrice { get; set; }
}

Each Car has a RentPrice. This price should be copied to Order's RentPrice when creating one. The car is selecting by user so initially Order.RentPrice is 0.

Here I want to copy price value:

[HttpPost]
public ActionResult Create(Order order)
{
    order.RentPrice = _context.Cars.Find(order.CarId).RentPrice;

    if (ModelState.IsValid)
    {
        _context.Orders.Add(order);
        _context.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(order);
}

It's not working because of an error on the SaveChanges that entity has validation errors. OK. I found that need first to call UpdateModel(order); and then change values.

So what I have. Working code:

_context.Orders.Add(order);
UpdateModel(order);
order.RentPrice = 777;
_context.SaveChanges();

Not working code:

_context.Orders.Add(order);
UpdateModel(order);
order.RentPrice = _context.Cars.Find(order.CarId).RentPrice;
_context.SaveChanges();

Working code (!):

_context.Orders.Add(order);
UpdateModel(order);
var t = (double)_context.Cars.Find(order.CarId).RentPrice;
order.RentPrice = (decimal)t;
_context.SaveChanges();

Can someone explain, please, what is going on here? Especially magic on the 3nd and 4th lines in the last block of code.

Update

I'm getting DbEntityValidationException: "Validation failed for one or more entities. See 'EntityValidationErrors' property for more details." From the inner exception: "OriginalValues cannot be used for entities in the Added state."