Half of the terrace sold to top floor flat owner?

Good morning to all. I have a question that a builder has sold half of the terrace top to the top floor flat owner. I have also complain to the PMC. it is also mentioned in his sale deed that the overhead terrace has been sold and also is mention in other flat owners agreement that the terrace will always remain with the proprietor and he can dispose it off. I would like to know if it is legal for the builder to sell half of the terrace and mention it in the sale deed and if any action can be taken against the builder of the flat owner Flat was sold in 2006. Thank you in advance ,