According to Ephesians 2:3 we were by nature deserving of wrath.

Then Christmas comes and rather than receiving what we deserve we receive that which we don't deserve Ephesians 2:4

Ephesians 2:4 But because of his great love for us, God, who is rich in mercy, made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved.

At Christmas, the coming of Christ, God rather than giving us what we deserved gave us something we did not deserve

At Christmas when God gave us what we did not deserve was He practicing Injustice?

Christmas started God's Plan that we would receive that which we did not deserve not as a plan to ignore the Law because Through Christ the law would be satisfied, justice would be served. Christ would be our substitute

At Christmas then; we receive that which did not deserve. This does not come to us as act of justice extended towards us nor does this come as an act of injustice by God when He did not practice justice towards us by giving us what we deserved. At Christmas God stepped outside the execution of justice not by practicing injustice towards Himself or us but rather practiced non justice by extending to us Grace rather than Justice and so giving to us that which we do not deserve.