Until end of turn, target creature gains ": Return target nonland permanent to its owner's hand."
The targeted creature’s controller, not Banishing Knack’s controller, is the one who can activate the creature’s new ability.
“Summoning sickness” applies. The new ability can’t be activated unless the targeted creature has been under its controller’s control since the beginning of that player’s most recent turn or it has haste.
: Return target nonland permanent to its owner's hand."