Yes. You may sign a preliminary sale agreement on your own; however, the property will be considered joint property if the purchase is completed before the effective date of the divorce, which is the date on which the divorce agreement is filed with a notary. Unless the spouses have agreed to retroactively apply the effects of the divorce to an earlier date (Article 262-1 of the French Civil Code). As a precaution, it is advisable to wait until the agreement becomes enforceable.