No, you are not required to sell, even if a prospective buyer offers the asking price (see, to that effect, Cour de cassation, 3rd Civil Chamber, 12 February 2008, No. 06-21.709, concerning a mandate given to an estate agent; and Cour de cassation, 3rd Civil Chamber, 17 June 2009, No. 08-13.833, concerning a mandate given to a notary).
As a rule, the intermediary does not have authority to sell on the owner’s behalf, unless the agency agreement expressly confers such authority (which is rare in practice).
However, if you refuse an offer at the asking price, the intermediary may seek compensation for the work performed, alleging a breach of contract. If payment is refused, the intermediary could attempt to recover such compensation through legal proceedings.