He can pardon him to test the effects on polling. If it helps, great. If it hurts him, get Snowden back, arrest him, and claim it was a trick to get the traitor that Obama couldn't get all along.
Is un-pardoning a thing? I'd have figured that a pardon would mean that Trump wouldn't be able to follow it up with his arrest and claiming the pardon was a 'trick'.
Just writing that, though, it feels dirtily and precisely "Trump".
You pardon certain crimes. As an example, you pardon Snowden for "all crimes related to NSA leaks," and then charge him with some kind if tax avoidance. It'd be dirty as hell, and make any trial much longer, but it's possible.
Just writing that, though, it feels dirtily and precisely "Trump".
edit: grammar