The "bon de visite"

This document is the proof that the agency has put the buyer in touch with the seller. If the buyer then contacts the seller directly, thus bypassing the estate agency ( for example, to avoid paying the agency commission), the bon de visite will serve as proof of the intervention of the agency and can be used in any legal proceedings to demand the payment of the commission. 

This is provided that the agency has signed a mandate with the seller and this has not expired or been terminated early.