Unless otherwise agreed in the employment contract or agreement, the employer cannot unilaterally waive the non-competition clause during performance of the employment contract.  

This is a logical solution since, in the past, the Cour de cassation specified that the employer could not unilaterally waive the non-competition clause where such a possibility had not been provided for by the parties in the clause (Cass. soc., 28th November 2001, n°99-46.032).