Trevor is an art dealer based in Melbourne who has been buying and selling art from different collectors in Australia. Arthur is an art collector based in Perth who regularly sells paintings to Trevor. As part of their business arrangement Trevor and Arthur apply a buy now pay later service whereby Trevor makes payment for purchased paintings after two months. On 1 November, Trevor appoints Silvia as his agent in Perth and that same day, he sends a letter to Arthur confirming Silvia's appointment and her authority to buy and sell pictures on his behalf. In fact, Silvia is under an express instruction not to deal with any pictures alleged to have been painted by Malevich, because of the risk of forgery. On 10 December, Silvia, purporting to act on behalf of Trevor, contracts with Arthur to buy from him 'Black Square', a genuine Malevich painting, for $60,000 - utilising the buy not pay later service. Silvia then secretly sells the painting, without informing Trevor and keeps the money for herself. Silvia later resigns as Trevor's agent and moves to New Zealand without informing Trevor about the purchase. On 12 February, Arthur sends a reminder to Trevor about his $60,000 bill for the painting. Trevor soon discovers what Silvia has done but has refused to make the payment claiming that Silvia was not authorised to buy any paintings by Malevich. What is the issue?