CRA indicates that no GST/HST should be charged on a cancellation fee paid by a new home builder to the purchaser
In an appreciating housing market, an individual purchaser whose plans have changed may assign his purchase contract back to the builder for an amount based on the appreciation to date, rather than selling his purchase contract to a third party.
On the assignment, the purchaser would be considered to be transferring an interest in a residential complex. If the purchaser was not a builder (which generally would be the case if he had entered into the purchase contract for the purpose of using the house for personal use rather than for resale), the general exemption for sales of interests in residential complexes would be available. If the purchaser instead was a builder, the assignment would be taxable - but still no GST/HST would be charged given that the builder/assignee would be registered.
Neal Armstrong. Summary of 23 March 2017 CBA Commodity Taxes Roundtable, Q.20 under ETA Sched. V, Pt I, s. 2.