Every man according as he purposeth in his heart, so let him give; not grudgingly, or of necessity: for God loveth a cheerful giver. Not grudgingly, or of necessity - The Jews had in the temple two chests for alms; the one was של תובה of what was necessary, i.e. what the law required, the other was של נרבה of the free-will offerings. To escape perdition some would grudgingly give what necessity obliged them; others would give cheerfully, for the love of God, and through pity to the poor. Of the first, nothing is said; they simply did what the law required. Of the second, much is said; God loves them. The benefit of almsgiving is lost to the giver when he does it with a grumbling heart. And, as he does not do the duty in the spirit of the duty, even the performance of the letter of the law is an abomination in the sight of God.
To these two sorts of alms in the temple the apostle most evidently alludes. See Schoettgen.