Writing your own contracts is perfectly possible, and legal. But it's also an incredibly bad idea. There's two reasons for this: Property law is complicated. Because it's such a fundamental part of legislation, it's often lots and lots of different laws layered on top of each other.
In other words, an assignment clause allows the buyer of a home to sell the place before they take possession of it. In Ontario, assignments are more common in pre-built homes and condos than on re-sale properties, but they are possible on any type of trade.