If I understand it good, your controller has to look like:

 

public ActionResult Edit(int id)
{
  Partner partner = db.Partners.Find(id);
  ViewBag.Sifra = new SelectList(db.Partners, "Sifra", "Naziv", partner.Sifra);
  return View(partner);
}

view:

@Html.DropDownList("Sifra", null, new { disabled = "disabled"})