Document the assignment outside France
If your French employer sends you abroad temporarily, specify the purpose, location, duration and people responsible for supervision. Assignment documents should make pay, travel and reimbursed expenses understandable; also discuss the conditions for returning to the company before departure so that both stages form part of the same agreed plan.
Confirm the framework applying to the assignment
A posting requires the situation and formalities to be checked with competent bodies. Coordinate social protection evidence and destination obligations, including those concerning work, with human resources; using the word posting in a letter does not by itself establish that every condition of that arrangement has been satisfied.