Effective date of service tax on 30% portion of Flats.

Dear Sir / Ma'am. I have booked a flat in Deepganga Housing, Haridwar on 01.01.2016. That time the s.tax was 14.50 % and applicable portion was 25 % (Thus tax @3.625 %). We paid the 2nd instalment on 23.04.2016 with same tax rate (3.625%). Now when the 3rd installment was due on 01.06.2016, the builder is asking for tax @ 4.50 % ( as tax is 15% and applicable portion is 30% by budget 2016). Apart from this..... he is demanding the rest amount as of tax on 2nd installment was collected less by him (calculation of tax @4.50 % on 2nd installment paid on 23.04.2016 the differential tax demand is of Rs 4600 along with the fresh demand of 3Rd installment. I need guidance on following - 1. Should I pay with tax rates prevailing at the time of booking or is it fine to pay tax applicable at time of scheduled payments every time? 2. What is the applicable date of 30% portion of tax, 01st April or 1st June 2016? If it is 1st April 2016, should I pay him differential amount for payment already made on 23.04.2016 taxed @ 3.625%? Please help