So they asked him, “Who is this fellow who told you to pick it up and walk?”
“What man is that…” is a contemptuous expression, “Who is the fellow?” (RWP) The authorities’ use of the term “man” is in contrast to the law of God. “How can a mere man order the violation of God’s law?” Note that they completely ignored the miraculous healing and dwelt solely on the breaking of the law.