(Allah's Statement): "... Thereafter (is the time) either for generosity or ransom (according to what benefits Islam)...

And the Statement of Allah عز وجل: "It is not for a Prophet that he should have prisoners of war (and free them with ransom) until he had made a great slaughter (among his enemies) in the land. You desire the good of this world (i.e., money of ransom for freeing the captives)." (V.8:67)