While justice and mercy are both employed by God, I fail to see how the two are applied simultaneously to the same individual. Mercy requires God to relent from doing justice, and not apply to the individual the justice their actions should require of them. God can remain just in doing so because in Christ He has already exacted justice. But I don't see how both justice and mercy can occur simultaneously.
I am still exploring this idea myself, but I don't see them as necessarily being in conflict.