Merciful Minion

Even the most hands-on villain can't take care of imprisoning or killing all the heroes personally. Ordering an underling to attend to such duties is common. Sometimes, the underling volunteers to "deal with" the hero ... and releases him instead.

There are several possible explanations. Perhaps the underling is a mole in the villain's organization. Perhaps the underling feels he "owes" the hero and chooses this form of repayment. Perhaps the underling is going through a Mook Face Turn, or simply expects the good guys to win and wants to score points toward a pardon.

Compare to If You're So Evil Eat This Kitten (where the villain orders an underling to "take care" of the hero as a loyalty test), "Alone with Prisoner" Ploy (which is just as likely to be information-passing as "time to escape"), Even Evil Has Standards, Minion with an F In Evil, and Noble Demon. Can overlap with Death Faked for You.

Examples of Merciful Minion include:

Anime and Manga

In the Arlong arc of One Piece, Nami pretends to stab Usopp to save him from the Fishmen.

Film

Force 10 from Navarone (sequel to The Guns of Navarone). Major Schroeder's mistress, Maritza, volunteers to take Mallory and Barnsby to recover the penicillin and to execute them if it doesn't exist. She eventually kills the German guards, reveals that she's actually a member of the partisans and lets Mallory and Barnsby escape.
In Hot Fuzz, Danny faked stabbing Nick in order to prevent the Neighborhood Watch Alliance from killing him for real.

Literature

In Les Misérables the situation is reversed, with the heroic revolutionaries deciding to execute the spy Inspector Javert. Jean Valjean requests that he be allowed to execute him personally as a reward for his services in the defence of the barricade. He takes him out of sight, fires a pistol into the ground and tells Javert to run.
In the first full Ciaphas Cain novel a riot breaks out on the ship he's assigned to. In the aftermath he can't have anyone executed (since it would destroy morale, and more importantly make him likely to suffer an "accident" on the battlefield) but the captain wants blood (he was in a relationship with one of the military police killed in the riot). So Cain lets the captain hold a tribunal (the defendants of which are found guilty) and sentences them to "death" by transferring them to a penal legion to die in battle.
Xanth Filatine from The Edge Chronicles. As a part of his Heel Face Turn, he shoots the rope tethering the protagonist's ship instead of into Rook himself, allowing Rook to flee from the Mooks swiftly closing on his location.
In Harry Potter and the Deathly Hallows, Draco refuses to admit the prisoner was Harry. Later, Narcissa crosses this trope with Death Faked for You and tells Voldemort that Harry is dead.

Live-Action TV

In Luther, Ian Reed pretends to be a dirty cop to get to a ritual killer for the protagonist. Painfully subverted later, though.

Oral Tradition, Folklore, Myths and Legends

Snow White's evil stepmother ordered a huntsman to kill Snow White and bring her heart back as proof. He leaves her in the woods instead, and brought back the heart of a deer.
- Which makes another one for the Evil Overlord List: Always ask the assassin to bring back the head.

Western Animation

In the 2011 ThunderCats reboot episode "Omens Part One", Lion-O shows mercy to two stockaded Lizard prisoners by convincing his father to set them free. In "Part Two", one of the two Lizards repays the favor during the Lizard invasion, by smuggling the key to Lion-O and Tygra's cell in some soup when they're Locked in the Dungeon, allowing them to escape.