Indlaw SC 681],though the principle was stated far more precisely,in the following terms: "The principle of waiver although is akin to the principle of estoppel; the difference between the two,however,is that whereas estoppel is not a cause of action; it is a rule of evidence; waiver is contractual and may constitute a cause of action; it is an agreement between the parties and a party fully knowing of its rights has agreed not to assert a right for a consideration