Its a toughie because when you sign a private purchase contract you havent actually bought a property, you have only bought the rights to buy a property and so it is actually this right to buy a property that they are trying to charge iva/vat on.
It can be understood capital gains, but not the iva which is effectively being paid twice and which many lawyers are saying the law passed does not apply to. It does apply to a resale.
The cancellation used to come in where big profits were being made and the difference usually paid in cash - it was really for the developer to show the taxman he hadnt sold the same unit twice but this was stopped to ensure tax evasion didnt take place.
I guess its to each how its challenged but I have known it where people stood their ground and it was sorted out, no double payment.