IIRC, you can also just drop slasher on the ground before talking to the orcs in order to keep it.
Note that if you did not install the
patch before starting the current game, Finnigan will not sell you back the magic lockpicks, so you will have to choose between completing the quest and keeping the lockpicks (or 'cheat' to replace them, see below).
Lost Items - guide and directory of inventory files: ignore the first reference to Finnigan's lockpicks, which directs you to a different topic, and see a reply further down. Since the latest patch, this is the best way to replace them.