Dude, dont even bothor returning Umbra, unless you dont want to give up a different item for the MQ. Personally, I give Martin Volundrung and keep Umbra. Dont go back to the shrine. As long as the quest remains active, Umbra weighs 0 lbs.

Who cares if you got Mehrune's Razor, Umbra's sweet anyways. Plus, what if the Razor breaks?