No objection certifcate

Hello Sir, I have purchased a resale flat in Mumbai and the flat is still under construction. Now builder is charging money from seller for NOC which seller is ready to give to builder. Now my concern is there any other document which is required from builder to transfer the ownership in their books. Reason being the building is still under construction and just wanted to confirm that there are no hidden charges by builder at the time of completion of flat since at that point seller will not be involved and there is a risk that I might have to bear those charges. Also what all precaution or document from Builder do I need to take before making final payment to the seller.