Thou Shalt (Not) Kill a Child: The Voice of Sacrifice

The mythical scene of Isaac's sacrifice introduces a question regarding the father-child relationship through sacrifice. Abraham is the father of monotheistic religions, the great father willing to commit filicide at God's command. God's first mandate was: Kill thy child, offer him up in sacrifice. Without hesitating, Abraham gets ready to comply with the command and only at the very last moment, when he is ready to behead the child, God's voice reappears and says (No). Both the prescriptive and the proscriptive mandates converge in a single event. What explains this internal contradiction of the divine voice? Why does it order Abraham to sacrifice his beloved son? What is the function of the child in the sacrificial act?