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"})