A person purchased an under constructed property in March'15. He paid 10% booking amount in March'15. ST was also paid on booking amount @ 3.03% (25% of 12.36%). The next installment became due in Dec'15. At this time, ST rate changed to 3.625% (25% of 14.5%). Now, the question is should the builder charge ST @ 3.625% or 3.03% on the amount that is due in Dec'15?