I find it hard to believe, to be honest.
Aren't there any ways in Australia to make selling of a house "atomic", with ownership transferred the same moment the money are transferred? Or they just move there without having ownership?
Sounds like something covered by tort law.
In NY, the standard is roughly that after signing a contract the buyer writes a check for 10% of the purchase price to the seller's attorney in escrow. This is typically done after a home inspection, but you still have mortgage and potentially other contingencies. If the buyer backs out for a reason not stipulated in the contract, the seller is entitled to the 10% escrow as damages - but that is not going to happen without litigation. If the seller tries to back out, the buyer will have a lien on the property.
Every state and every transaction can be different, but once you get far enough along, it is generally in your interest to have the deal close. If not, you're going to start a months long process pretty much from scratch.
Someone asking for an addition though would be well beyond what is reasonable; and I can't imagine any seller actually agreeing to that.