Can builder forefiet the entire booking amount?

I booked the property in Pune on 30th June 2017, based on the confirmation by the sales person that there will not be much impact of GST as there willl be input credit to them and the price quoted will remain same and may vary by few thousand rupees only, so at the time of booking I paid 1Lac rupees as token booking amount, during application signing the sales people were in hurry to attend some training & requested us to sign the documents stating that it just an application form and nothing to worry about it, we in good faith signed the documents, later builders CRM team sent me agreement for the agreed prices and when we were ready to sign the agreement suddenly within two days asked me to stop and sent me revised agreement with higher cost stating it is because of GST effect which was beyond my budget, so I requested for cancellation of booking for which the CRM team asked to submit the application on 100rupees stamp paper, though I submitted the request now they are saying they will deduct the 20% of the amount which is not justified. They also did not give me the signed documents for my records, now when I could see the documents it mentions complete amount will be forfeited Can builder do this? What remedies do I have?