Article L1222-6 of the French Labour Code
When the employer envisages modifying an essential element of the employment contract for one of the economic reasons set out in article L. 1233-3, he shall propose this to the employee by registered letter with acknowledgement of receipt. The letter of notification informs the employee that he has one month from receipt to make known his refusal. The deadline is fifteen days if the company is in receivership or liquidation….