Deed of apartment

Executed agreement to sale for purchase of resale property in a condominium. property owner initially agreed to provide all the legal documents. However, just before agreement to sale, it was told that deed of apartment was not executed by the builder. As the owner was in hurry (going to US) he told me that builder will execute the deed of apartment with you. Builder is also agreed to do so. And accordingly clause was mentioned in agreement to sale. Now I understand that builder has no right to execute deed of apartment as he has already executed deed of declaration and he has no rights. Flat owner is not willing to execute deed of apartment as he says that builder has not executed the same with him. I have paid 20% of total consideration to owner at the time time agreement to sale. will it be OK to execute the deed of apartment by the builder directly with me (I am not the first owner. ? Or can first owner execute the deed of apartment even though builder is not executed with him?