What is the test for an agent to be paid commission for completing a sale?

Kerans J. goes on to adopt the remarks of Davies J. in Stratton v. Vachon (1911), 1911 CanLII 50 (SCC), 44 S.C.R. 395 at 401: The knowledge on the part of the vendor that the person with whom he completes the sale was introduced by the agent is not the test of his liability to pay commission, but the fact whether the agent’s acts have really been the effective cause of the sale, and if the agent’s acts have brought a person or persons into relation with his principal as an intending purchaser, and the sale is effected, the agent has done what he contracted to do and is entitled to be paid.