I am selling my house. My notary sent the declaration of intent to alienate (DIA) to the town hall. If the latter decides to exercise its right of pre-emption, does it have to acquire the property at the price initially fixed?

No. A municipality can very well decide to pre-empt a property offered for sale at a selling price lower than that desired by the seller.

The latter must send the owner its proposal by registered letter with acknowledgment of receipt, within 2 months of receiving the DIA.
The seller then has a response period of 2 months (article R213-10 of the Town Planning Code) to:
- accept the price proposed by the municipality,
- or renounce the sale of his property.
In the event of a disagreement on the price, the seller or the municipality can take the expropriation judge (at the court of law) to have the price set judicially (article R213-8 of the Town Planning Code).