Article 897 of Civil Code

1 - Anyone wishing to exercise pre-emption shall formally declare his intention to both the seller and the purchaser within fifteen days from the date on which he learns of the sale, failing which his right shall lapse. The formal notice provided for in the preceding article shall constitute conclusive evidence of knowledge. 2 - The declaration of intention shall not be effective against a third party unless he knows of it. If the thing sold is an immovable, registration of the declaration of intention shall constitute conclusive evidence of the third party's knowledge.