There are some instances where the agreement of sale lapses, and does not need to be cancelled.
This happens when the sale is subject to a suspensive condition (for instance, the approval of a mortgage bond, or sale of the purchaser’s property) to be fulfilled within a certain time period, and that time period expires without the condition being fulfilled.
In this instance, no cancellation is necessary, as the agreement of sale automatically lapses.
Once the suspensive conditions have been fulfilled, and a valid and binding agreement of sale has come into existence, then cancellation would need to take place if you no longer want to proceed with the sale. In this instance, it is recommended that you take advice from a property law expert.
Bear in mind that if you are the purchaser and the sale is subject to you obtaining a mortgage bond or selling your existing property, you are obliged to do whatever is reasonably possible in order to obtain such mortgage bond or sell your property, and cannot rely on the lack of fulfilment of the suspensive conditions in order to withdraw from the sale.
Unlawful cancellation may bring about a damages claim, and entail lengthy and costly litigation, so it is always a good idea to get advice from a property lawyer / conveyancer.