Treat your enemies with courtesy, and you’ll see how valuable it really is. It costs little but pays a nice dividend: those who honor are honored.

[Tiénese por deuda entre enemigos para que se vea su valor. Cuesta poco y vale mucho: todo honrador es honrado.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 118 (1647) [tr. Maurer (1992)]
    (Source)

(Source (Spanish)). Alternate translations:

[Civility] is even a duty, and in use amongst Enemies, which shews the power of it. Whoever honours is honoured.
[Flesher ed. (1685)]

Between opponents [courtesy] is especially due as a proof of valour. It costs little and helps much: every one is honoured who gives honour.
[tr. Jacobs (1892)]

Hold to [courtesy] as a matter of duty between enemies, for it exhibits our courage, costing little, and being worth much, for to show honor is to be honored.
[tr. Fischer (1937)]