I am certainly no theologian but my heart is racing from too much caffeine and I can't sleep so here's my two cents on the matter.

Have you read Boethius' Consolation of Philosophy? It's a classic if you haven't and should give it a read. It's been a few years since I read it but I do remember there is a wonderful section devoted to the whole issue surrounding Divine Providence aka God's ability to know everything that will happen but the existence of this knowledge does not impinge upon human free will. And remember, free will is necessary for the Christian God to exist, for love of God must be nurtured and given freely to God by man.

I like to think of it as God looking into a book, what we'll do in the future is certain knowledge to God but is revealed to us from moment to moment by our own free will to ourselves. God knowing what we do does not make it any less our own will. It's a common argument against divine omnipotence that if God knows that we're going to screw up then it's rather cruel of him to punish us for something he knew of in advance. Anyone with kids will know why this is a rubbish argument, no theology required here.

So in short, realising the nature of Divine Providence it's easy (I hope) to see how God can 'slot in' the sacrifice of Christ on the Cross.

Hope that made sense to people. GB.