Is parol evidence admissible to challenge a mortgage agreement?

As to the case of Morrison v. Cybulak, supra, while it is no doubt established law that where a document purports to be the final record of the agreement of the parties, parol evidence is generally inadmissible to contradict or vary its terms, yet it is equally established law that parol evidence is admissible to show that a written document which purports to be unconditional was in fact executed with the intention that it should only take effect as a contract upon the performance of a condition precedent, or was not intended to operate from the time of its execution, but from some future uncertain time. If the defence in the above case was, that the mortgagor and mortgagee at the time the mortgage was made, agreed that it was to become operative only in case the wife sold the land, evidence of such agreement was, in my opinion, admissible.