 Consequently, evidence may be given of a subsequent oral agreement altering or cancelling the written agreement, except where the contract is required by statute to be in writing, for such a contract cannot be varied by a later oral agreement, though it may be cancelled by such an agreement.Similarly, where the contract itself provides that it can be varied only in writing an oral variation is void, and so too, it seems, is an oral agreement to cancel the contract.The rule excluding oral evidence derives not from the Roman-Dutch law, but from the English law of evidence, which has been adopted throughout South Africa by legislation.