"You have heard that it was said... But I say to you..."
This is a formula that Jesus uses repeatedly in the Sermon on the Mount. In this formula, Jesus quotes a teaching from the Law of Moses and then states his own teaching. What is Jesus accomplishing with this formula, though?
At first glance, it can look like Jesus is undermining the Law of Moses, but that misses the point of what Jesus is actually doing. One commentator writes that "Jesus' proclamation of the will of God is the 'door' to the Old
Testament." It isn't that Jesus is undermining the Law of Moses, more so as open up the Law of Moses, and that values holding it up, to us.
Dale Bruner writes that, in Jesus' time, "contemporary teachers often said, 'But I say to you,' modestly, to contrast their remarks with those of other human teachers. But Jesus cites the OT Word of God . . ." With this understanding in mind, this formula isn't meant to lower the status of the Law of Moses, but to elevate Jesus' status as the authoritative teacher of the Law of Moses. Jesus doesn't cast aside the Law of Moses, so much as he explains the Law of Moses. And he is the only one who can adequately explain God's intent behind the Law of Moses.