So also the devil is said to torment men justly, because God in justice permits this, and man in justice suffers it. But when man is said to suffer justly, it is not meant that his just suffering is inflicted by the hand of justice itself, but that he is punished by the just judgment of God. But if that written decree is brought up, which the Apostle says was made against us, and cancelled by the death of Christ; and if any one thinks that it was intended by this decree that the devil, as if under the writing of a sort of compact, should justly demand sin and the punishment of sin, of man, before Christ suffered, as a debt for the first sin to which he tempted man, so that in this way he seems to prove his right over man, I do not by any means think that it is to be so understood. For that writing is not of the devil, because it is called the writing of a decree of the devil, but of God. For by the just judgment of God it was decreed, and, as it were, confirmed by writing, that, since man had sinned, he should not henceforth of himself have the power to avoid sin or the punishment of sin; for the spirit is out-going and not returning (est enim spiritus vadens et non rediens); and he who sins ought not to escape with impunity, unless pity spare the sinner, and deliver and restore him. Wherefore we ought not to believe that, on account of this writing, there can be found any justice on the part of the devil in his tormenting man. In fine, as there is never any injustice in a good angel, so in an evil angel there can be no justice at all. There was no reason, therefore, as respects the devil, why God should not make use of as own power against him for the liberation of man.
When Boso speaks were are not to presume that this is the doctrine advanced by St. Anselm. This installments generally, is a large array of objections, to which St. Anselm only begins to answer in the end. All that has been said so far is that the dual nature of Christ makes it possible for God to not command His Passion. I suggest we wait till St. Anselm actually addresses the putative contract with the devil issue. We know that he offers a doctrine different from the ransom doctrine, but so far it has not been addressed.
I am not sure if Catholics use the term "efficacy" either. I was merely struggling to define Unlimited Atonement, which we teach and I know the Orthodox Church teaches. Further, the Catholic Church does not reject the Ransom doctrine, even if it also teaches the atonement by satisfaction. We consider all patristic atonement theories, up to and including Anselm's, theological speculation, as far as I know.
I don't think anything in the text suggests an opposition to the Ransom doctrine, except the remark by Boso that surely God did not owe anything to the devil, and you agreed with that.
We do. God provides salvation, but it is not "efficient" if it is thrown away by the individual. We, Orthodox and Catholic, believe that we have some say on whether we will accept God's offer or not.