If it were me, I'd just give him the $50 bill and he would soon tell me if he couldn't change it himself. He might then ask me if I had anything smaller or offer to change it, himself, via a nearby shop/bank (as he wants to achieve a sale). But then, that's just me!
With regards to the "English" part of your query, I would have probably asked: "Can you change a 50-dollar bill?" (i.e. "Are you able to change ....").
Hope this helps.
Student or Learner