Siddha Town, Rajarhat takes excess money for interest on late payment, not admitting the fault.

Q: When I made the last payment, they have taken approximately Rs 33,000 extra with no basis at all.
When I corrected Rs 15,000 at their office, they show the balance on the possession letter, but has not repaid it. I found another wrong calculation of Rs 18000 and when I claimed it over phone, they said that they would look into the matter. I called them several times, every time they refer the matter to some other lady who knows nothing about it. Even when I sent my sister with documents, she denied to look into it.
Is she trying to fraud with customers? I have seen similar complains against her in the forum. If I pay the registration money I fear they would not return the money. I have enough proof on papers. I stay abroad. What kind of legal action can be taken?