From Charles Spurgeon's The Treasury of David, on Psalm 120:3 - what shall be given to thee? (the person who slanders).
What is the expected guerdon [reward] of slander? It ought to be something great to make it worthwhile to work in so foul an atmosphere and to ruin one's soul. Could a thousand worlds be bribe enough for such villainous deeds? The liar shall have no welcome recompense: he shall meet with his deserts; but what shall they be? What punishment can equal his crime? The Psalmist seems lost to suggest a fitting punishment. It is the worst of offences—this detraction, calumny, and slander. Judgment sharp and crushing would be measured out to it if men were visited for their transgressions. But what punishment could be heavy enough? What form shall the chastisement take? O liar, "what shall be given unto thee?" Or what shall be done unto thee, thou false tongue? How shalt thou be visited? The law of retaliation can hardly meet the case, since none can slander the slanderer, he is too black to be blackened; neither would any of us blacken him if we could. Wretched being! He fights with weapons which true men cannot touch. Like the cuttlefish, he surrounds himself with an inky blackness into which honest men cannot penetrate. Like the foul skunk, he emits an odour of falsehood which cannot be endured by the true; and therefore he often escapes, unchastised by those whom he has most injured. His crime, in a certain sense, becomes his shield; men do not care to encounter so base a foe. But what will God do with lying tongues? He has uttered his most terrible threats against them, and he will terribly execute them in due time.