A few years ago, my mother donated a real estate property to me. The deed included a clause de retour conventionnel (right of return) in case I were to die before her. She has now passed away, and I wish to sell the property. Is this possible?

Yes. The clause de retour conventionnel is a resolutory condition of the donation, as provided under Article 951 of the French Civil Code. It allows the donor to recover the donated property if the beneficiary dies before them. Since your mother has passed away, the condition can no longer be triggered. Therefore, you are free to sell the property. You will simply need to provide proof of your mother's death to the notary handling the sale.