D.D is not a cheque but almost cash, so there is no need to mention "subject to realization". In any case if you are apprehensive that the D.D handed over to you might not be encashed, you can include one clause in the sale deed that if full sale consideration is not received or the negotiable instrument gets dishonoured for whatever reasons, then the sale deed is null and void.
this should protect and safeguard your interest.