I have witnessed PSA presented in such a way as to validate the objection of Socinus (i.e., "divine justice demands that..." or "God first had to..."). If God had to receive payment in full in order to forgive or risk violating His own sense of "justice", then that is not forgiveness. The fallacy of that method of argument is that PSA itself is misrepresented by such wording.Let's look at this issue from the other direction, that of Socinus.
God is not a slave to His nature, but instead is revealed by that nature. God's "justice" did not have to be met, nor did God have to receive payment in full in order to forgive (that would not be "forgiveness"). God Himself taking our sins and purchasing us with his own blood is not merely paving the way so that He can forgive us. It is the basis of forgiveness itself (we were purchased with a price). It goes back to that multifaceted gem. God's redemptive plan was in such as way so that He is both just and the justifier of those who are saved.