heat. Front window ordered – but not delivered – retrofitting possible?

Hello together! With our new C-Max the special equipment “Heatable windshield” ordered by us was not delivered. According to the dealer a retrofit is technically not possible and it therefore offers us in addition to refund of the surcharge (self-evidently) a discount still to be negotiated… – How far can I trust the statement regarding the not possible retrofit? – What a discount is possible or usual in such cases? Thanks for feedback! Mar tin