Where there is documentary evidence on an aspect of a party’s case, no oral evidence is admissible on that aspect. This is because our adjectival law does not admit oral evidence on an aspect or area covered by a document. A party cannot benefit from two ways: documentary evidence and oral evidence. He can only lead evidence in respect of one and not the two of them. But this principle of law is subject to an important qualification and it is this. If the parties by their ad idem agree by oral agreement to change part of the written agreement, the court will not reject the oral agreement.
– Niki Tobi, JSC. Brossette v. Ilemobola (2007)