Rate issue

Hello, I bought an office space in an under construction project in Noida Sec140 in July 2014. I wanted to go for an investment so i discussed rates and everything with the VP-Sales. After negotiations we agreed to go for 5400/psf with a payment plan of 60:20:20 rate where as the offered rate was 8000/psf. and Immediately I paid little more than 20%(to make it round figure value) of the total amount. After a month I got a demand letter asking for payment for remaining 40% of the First 60% payment which was as per the 7000/psf. When asked, VP-sales said there must be some miscommunication, you pay according to the demand letter, we will adjust the amount in the next payment. I paid that amount and asked her to keep Sale agreement ready. She started avoiding me and started not replying my call and message. After trying & waiting for months I visited developer's Head office in Noida where I came to know that the company which sold me the property was an outsourced company which works for the developer and everywhere the rate mentioned was 7000/psf. I asked them to return my money because i can not pay so huge amount but they said no. After so many arguements and discussion they agreed to provide us a rate of 5500/psf however they are not ready to mention it in the sale agreement neither on any other paper. A relationship manager said that he will send me an email of the detailed break up of the payment as per 5500/psf rate. But I am not convinced because I don't think it is a proof. I am keen to take my money back. Please advise me on the same.