If you are asking under what conditions you can cancel or seek to reverse payment for a draft, ask your bank manager.
I am not legally qualified, but I think your solicitor would advise you not to exchange drafts in this way, as it gives you little or no protection should the vendor decide not go go ahead (they have already delayed closing so their pulling out is not completely unlikely.). Your solicitor would also advise you that what you propose is tax evasion.
Does the contract exclude contents? That suggests the vendor may own the contents even after closing, so the vendor is protected (or at least, more protected than you would be if you gave them a bank draft a few days before intended closing). I would call their bluff on this.