I would suggest that the Father’s will was the death of the Son and the Son knew that was what God's (Father) will was and it was His (Son) will. There is no discussion. The will of the Father and the Son was one of the same. Your verse points that out.
I was just reading about + Elizabeth Seton and one of her big concerns was, "First: we should do what God wants, second: we should do it in that manner in which He wants it done and third (here's the kicker) We should do it because it is His will."
Here her standard is obedience, not because it's good for you to exercise and eat well (though it is) but because God wills it. Our obedience comes from our Love (which comes from grace and all that, but that's another part of the discussion.)
Anselm is saying that God the Fathers plan was essentially to acquiesce to the will of the Son, who agreed to do the Fathers will.
Okay, treat me like the idiot I am, please. Is it as though there were this conversation:
Father: I want to save the WORLD.or is it, more like:
Son: Well, I want what you want, whatever it is.
Father: Well I think the way we do it is You die for their sins.
Son: Okay.
Father: I want to save the WORLD.Or is this just so off base that I should go out and jump off a cliff?
Son: Well, I want what you want, whatever it is, and if you wanted me to die for their sins, I'd even do that.
Father: Well then, I think that's the way we're going to do it. We'll do it that way.
I would suggest that the Fathers will was the death of the Son and the Son knew that was what God's (Father) will was and it was His (Son) will. There is no discussion. The will of the Father and the Son was one of the same. Your verse points that out.
I think your post was clear enough, and I agree.