Currently you are not retrieving the additional value "bolag" in your action. I guess your model does not have property named "bolag". You can either add that or you can add additional parameter to you action like so:
[HttpPost]
public ActionResult Returer(ReturerDB retur, string bolag)
{
if (ModelState.IsValid)
{
db2.ReturerDB.AddObject(retur);
db2.SaveChanges();
return RedirectToAction("Returer");
}
return View("Returer");
}
After that, the selected value from the dropdown should automatically be in the bolag-parameter.
Edit.
You also need to add name-attribute to your select:
<select id="bolag" name="bolag"/>