Your professor probably also mentioned that there must be intent to make a binding agreement. If these agreements are regularly made in this situation merely as an offer to negotiate an actual contract at a later time, then the parties do not intend to be bound and they are not contracts. I believe that to be the case.

From Anderson's Business Law:

> Because a contract is based on the consent of the parties and is a legally binding agreement, it follows that the parties must have an intent to enter into an agreement that is binding. Sometimes the parties are in agreement, but their agreement does not produce a contract. Sometimes there is merely a preliminary agreement, but the parties never actually make a contract, or there is merely an agreement as to future plans or intentions without any contractual obligation to carry out those plans or intentions.

> In some cases, the fact that important terms are missing indicates that the parties are merely negotiating and that a contract has not been made. When a letter leaves many significant details to be worked out later, the letter or printed matter is merely an invitation to negotiate. It is not an offer that may be accepted and a contract thereby formed.