I'm about to buy a property. The property was rented, and the seller has given his tenant notice to sell. In the meantime, he has lowered the price by 20,000 euros. The notary tells us that we have to propose the sale of the property to the tenant again.

Is this true? 

Yes. Article 15 of the French law of July 6, 1989, stipulates that "in the event that the owner decides to sell on terms or at a price more advantageous to the purchaser, the notary must, if the lessor has not previously done so, notify the tenant of these terms and price, failing which the sale will be null and void". This condition is essential to guarantee the legal security of your purchase, since failure to comply with the obligation set out in this article could result in the tenant invoking the nullity of the sale.