If I understand him right, I think Cardinal Kaspar’s point is that mercy surpasses and indeed fulfills what we have understood as the justice of God. In fact, God is mercy. The Old Testament shows what we as a rebellious people deserve, and the New Testament shows God’s surprising solution. This solution, Christ’s incarnation, life, death and resurrection, is foreshadowed in the Old Testament, but only fully realized in the New. Some quotes from Cardinal Kaspar:
“There is a widely held opinion that the God of the Old Testament is a vengeful and angry God, while the God of the New Testament is a gracious and merciful God. Now there are, in fact, texts in the Old Testament that can support this position… Nevertheless, this view does not do justice to the gradual process by which the Old Testament’s idea of God is critically transformed, nor does it do justice to the internal development of the Old Testament in the direction of the New Testament. Ultimately, both Testaments give witness to the same God.”
“Mercy must be understood as God’s own justice and as his holiness. Only in this sense can we make the image of the good and merciful father, whom Jesus proclaimed to us, shine again… Mercy expresses God’s essence, which graciously attends to and devotes itself to the world and to humanity in ever new ways in history. In short, mercy expresses God’s own goodness and love… The determination of mercy as the basic attribute of God has consequences for determining the relationship of mercy to God’s justice and omnipotence. If mercy is the fundamental attribute of God, then it cannot be understood as an instance of justice; on the contrary, divine justice must rather be understood from the perspective of divine mercy”
From Mercy: The Essence of the Gospel and the Key to Christian Life, by Walter Kaspar.