Need to understand few things here, Is the property RERA registered? Have you obtained possession? Is additional amounts mentioned anywhere in the builder buyer agreement?
After getting answers to the above questions, I can assure you that no builder can obtain any unjust or unfair amount from consumer . On the other hand you can claim compensation for harassment.
For further clarification contact: email@example.com
Sir i can advice you something, but not here.
For any further assistance, feel free to contact me on my e-mail "firstname.lastname@example.org"
Shubham Pabbi & Associates