Ok missing a lot here... How long have you been riding this bike? Are you wearing a backpack? Commuting or just riding. Do you really think the bike is fitted correctly? 2nd opinion from another LBS.
Yeah I get the same thing too on my urban bike and my road bike. Quickest relief for me is get upright and and hold the bar with your fingers lightly wrapped around the bars, get your palms off of it. The numbness should go in a minute or two at the most.
I have a weak upper body and am still trying to isolate the exact cause, not sure if I am tensing up shoulder blades or traps area or if nerves are getting pinched in my neck. It is definitely worse when my backpack is heavier. I really try to watch my posture not allowing my torso to "slip through" or down out of my shoulders.
KEEP YOUR BIKE, this will happen when you ride other bikes as well. If you love riding you'll figure out a way to fix or deal with it.