Amount per square yard being raised by builder after 4 months

I booked a residential plot in a HMDA approved layout in Dec 2017 from a registered real estate company in hyderabad by issuing Account payee cheque for Rs.20,000 as advance and got receipt for the same duly mentioning the Plot No., Name of the venture etc. and on my request it was mentioned on the reverse of the receipt that the amount will be refunded if any problem in the layout and rate per sq.yard as @Rs.4000/- but no time schedule was mentioned. The cheque was realised by them. At the time of booking of the plot they said that their project is getting Bank approval soon and bank loan can be availed but and after getting bank approval after 4 months of my booking they say the rates have gone up and you have to pay Rs.4,500/- per sq.yard instead of Rs.4000/- else take back your advance as you have delayed 4 months in getting registration. Please advise me how to proceed further to get the plot registered in my name with the agreement amount of Rs.4000/-.