I rebuke a unit at one battlefield, he reacts and moves that unit to another battlefield. Rebuke resolves because he moved to a battlefield or is a mistargeted?

Not a mistarget — Rebuke resolves and returns the unit to hand.

Rebuke targets "a unit at a battlefield." Moving that unit to a different battlefield doesn't stop it from being "a unit at a battlefield," so it still meets the targeting restriction when Rebuke resolves. The target relationship persists as long as the unit remains on the board — only leaving the board (e.g., to base via Flash, or to hand/trash) would make it an illegal target (Rule 359.3.e.3; FAQ on targeting relationships). Moving battlefield to battlefield keeps it fully legal.