Builder buyer agreement

I booked a flat in a builder's project in Greater NOIDA, Uttar Pradesh. It was supposed to have been delivered by March 2017. However, only 20% of the construction has been done so far. I have already paid around 25 lac to the builder. On my request, the builder agreed to shift me to another project of his which is 70% complete. He promised to hand over the switched flat by December,2017.However, the builder-buyer agreement for the new flat says that the flat will be handed over within 3 years of the date of execution of the agreement. When I asked the builder to correct this position, he stated that he cannot deviate from the standard builder-buyer agreement, but he would give me in writing on the company's letter head that the flat will be delivered by December, 2017. My questions are- 1. Will the position conveyed through a separate letter on the company's letter head be valid in the eyes of law? 2. Should the position be conveyed on a stamp paper? 3.The Builder-buyer agreement has been prepared on a stamp paper of Rs.100. Is this fine? Is any stamp duty required to be paid at this stage? Does the agreement need to be registered? 4. Should I hand over the original builder-buyer agreement of the old flat to the builder which he has been asking for ? Will be grateful for advice.