“To a surrounded enemy, you must leave a way of escape.”

1950s, Loving Your Enemies (November 1957)
Context: Another way that you love your enemy is this: When the opportunity presents itself for you to defeat your enemy, that is the time which you must not do it. There will come a time, in many instances, when the person who hates you most, the person who has misused you most, the person who has gossiped about you most, the person who has spread false rumors about you most, there will come a time when you will have an opportunity to defeat that person. It might be in terms of a recommendation for a job; it might be in terms of helping that person to make some move in life. That’s the time you must not do it.

Source: The Joke

This is sometimes attributed to Sun Tzu in combination with the above quote, as well as alone, but it too has not been sourced to any published translation of The Art of War, though it is similar in concept to his famous statement in Ch. 3 : "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles..."
Misattributed

Source: The Devil's Right Hand

Go Rin No Sho (1645), The Water Book

At Lotos Club, January 10, 1885, quoted in [18422, Modern Eloquence: Vol III, After-Dinner Speeches P-Z]

Source: The Tales of Alvin Maker, The Crystal City (2003), Chapter 1 “Nueva Barcelona” (p. 19).

Source: The Bell Jar

Source: Culture and Value (1980), p. 50e