We have been married under the regime of the legal community for 3 years and we are considering a separation of property. I am going to negotiate a contractual termination of my employment contract and I wish that my severance payment doesn't enter...

into community. If it is paid after our change of plan, will it be considered as a clean property?

No. The severance payment is used to compensate for the loss of job loss and therefore enters the community (Article 1401 of the Civil Code). Consequently, except to repair a specific injury to the person of the employee (Article 1404 of the Civil Code: moral damage ...), it is a common good.

Moreover, according to settled case-law, the Court of Cassation admits that this indemnity arises on the day of the notification of the breach (Cass, soc, 19 January 1994, Cass civ, 1st, 15 November 2017). The deferred date of payment is irrelevant.