Ummm....things above the collar bone can be tricky...there are a lot of critical anatomic structures in that area......big blood vessels/top of your lung/brachial plexus.
If have arm tingling, is probably affecting brachial plexus (major nerves to your arm). Could be a growth on the nerve itself or some other space occupying growth exerting pressure on the nerves.
Am surprised that an MRI doesn't show it. Fat shows up nicely on an MRI and wouldn't expect that to be the issue anyway.
May want to have your MRI results revisited to make sure a proper look (contrast or not maybe?) of area was achieved.
Personally, I wouldn't let anybody cut on it till I knew what it was most likely and where exactly it sat.....then I'd probably submit to a needle biopsy rather than an incision if necessary. You don't want anything in that area accidentally injured.
Hope this helps,