You guys haven't been able to properly appraise the contents of the treasure room, but the instrument Vex has just offered is one of the non-magical pieces from that haul. It's listed as "ebony lute inlaid with dragonbone and set with blue diamonds." It's a masterwork musical instrument, once you get past the pimped-out decorative elements.
Vex thinks that Narg's tastes probably are fairly simple, so that good food, drink, or enjoyable/useful objects are more likely to get his attention than valuables like gold or gems that he can trade for practical goods. Because frankly, where is Narg going to spend cash if you give it to him?
Wæs se grimma gæst Grendel haten,mære mearcstapa, se þe moras heold
Anyone who wants to jump in to help Vex can still contribute via Aid Another. Make a Charisma or Diplomacy check, and post in the IC thread with some remark that you think might persuade a minotaur that it'd be a fair trade to swap a lute for one to three days' labor.
I'll advance In Character, shortly. Currently the minotaurs are willing to work for you for a day in exchange for this lute.
I guess the next question is whether you guys are going to try to talk Narg and his bros into sticking around, and if so, whether they get to share space in the cavern with you.