Had it not been for a previous ordainment from Allah, a severe penalty would have reached you for the (ransom) that ye took.
Yusuf Ali writes,
Though any motive of worldly gain, which may
have been in the minds of some among the victorious
Muslim army, is condemned as worthy of a severe
penalty, what actually happened is ascribed to the Plan
of Allah, which was preordained.
Among the prisoners taken were the Prophet’s uncle
‘Abbas and Hadhrat ‘Ali’s brother, Aqil, who afterwards
became Muslims. ‘Abbas was an ancestor of the founder
of the celebrated ‘Abbasi Dynasty which played such a
notable part in Islamic history. In his case the promise
made in verse 70 was amply fulfilled.
In the case of all prisoners, if there was any good in their
hearts, their very fight against Islam and their capture
led to their being blessed with Islam. Thus does Allah’s
Plan work in a marvelous way, and evolve good out of
Yusuf Ali, Abdullah. Translation and Commentary of the Quran. (surah 8 verse 68). Microsoft Word – 008 Anfal.doc (quran4u.com)