In Marbury v. Madison, the Supreme Court in an opinion written by Chief Justice John Marshall, declared an act of Congress unconstitutional. This was a first and from that day forth (February 24, 1803) the doctrine of Judicial Review was established. It is interesting to note that Thomas Jefferson declared that the concept of Judicial Review was unconstitutional but the decision remains unchallenged.
An agreement that is writing and signed by both parties makes it binding on both of them hence it is enforceable by the law. So an agreement to seek an interest in real estate must be put down in writing also so that it can be enforceable by the law should problem arise.