Adam Clarke on Psalms 37:21

The wicked borroweth - Is often reduced to penury, and is obliged to become debtor to those whom he before despised.

And payeth not again - May refuse to do it, because he is a wicked man; or be unable to do it, because he is reduced to beggary.

But the righteous showeth mercy - Because he has received mercy from God, therefore he shows mercy to men. And even to his enemies he showeth mercy, and giveth; his heart being disposed to it by the influence of Divine grace and his hand being enabled to do it by the blessing of God's providence.