Commenting about someone's country of origin in response to a sale seems offtrack. Without giving an unwarranted dissertation, I would think you could have simply said "As a proud U.S. based company, our policy is to boycot any involvement with French businesses. Thank you for your interest." If he wrote back asking why, or otherwise criticizing you, that is an open door to comment further (even verbatim with what you originally wrote). Outside of that, it seems entirely misplaced to send that as a reply to any French businesses (even one) that may contact you about another matter entirely. Your response ends up making your company appear more negatively. That's my opinion.
Regards,
~ Nexus