DONKOR v. WIH AND ANOTHER Evidence—Parol evidence—Admissibility—Circumstances in which parol evidence admissible on written document—Exhibit B executed by W and D indicating assignment by W of his interest in S.H.C. estate house to D—Action for specific performance by D—W claiming in defence that transaction really a loan and prior collateral oral agreement postponing operation of exhibit B for two years—D's own behaviour in relation to house contrary to exhibit B—Whether parol evidence admissible on issue.