Quest \"Bargain! Bargain!\" (Borderlands: The Pre-Sequel)

To complete the quest you need to piss off peaceful (green) animals, so they broke through the wall of the building.
This is given a special gun. But, this gun is very easy to sell, if you do not know that it for business needs.
How to get the quest after the sale of the gun? Or how to get it back (redemption at the dealer is not an option)?