What is the test for retaining a lien against a vendor in a commercial real estate transaction?

Boulton v. Gillespie, supra, at p. 228 (This opinion given by the trial court was endorsed on appeal at p. 233.): … if he can shew that the retention of such lien would defeat or even materially interfere with the known object of the purchase, so as to clog it with difficulties which it is reasonable to conclude that the parties could not have intended that the purchase should be incumbered with; then I think he rebuts the vendor's prima facie equity and establishes a state of things under which the retention of a lien would be the reverse of equitable.