Pay sales tax difference to builder ?

I had purchased a flat in Oct-2015 for which received possessation in Mar-2017. Now as the building was getting completed i was releasing the funds with the help of Bank. Now Builder is asking for the sales tax difference on each amount paid time to time. e.g. In Oct-2015 : Paid 20 Lacs (when sales tax was 3.55%) In Mar-2016 : Paid 10 Lacs (when sales tax was 3.75%) In Oct-2016 : Paid 10 Lacs (when sales tax was 3.85%) In Mar-2017 : Pad 15 Lacs (when sales tax was 3.95%) Now everytime sales tax was getting increased, but the overall salestax is already paid at the time of registration. Now builder is asking for the difference in sales tax for which he is not ready to provide the receipt. If look at above calculation, then he is demanding sales tax as given below. For Mar-2017 : Rs. 2000.00 For Oct-2017 : Rs. 3000.00 For Mar-2017 : Rs. 6000.00 So he is demanding Rs. 11000.00 So my question here is, taking the sales tax difference at the time of possassion of the flat is leagal ? I did asked for proper explanation on the section / norms which specify flat owner to provide sales tax difference but he is not providing that information too.