Mr. Chung was leaving Hong Kong and wanted to sell his apartment in Stubbs Road as quickly as possible. He asked Richard Ellis Limited, the estate agents, to find him a buyer. Mr. Chung signed an agreement with the estate agent undertaking to pay a commission of 0.5 per cent of the purchase price when the flat was sold.

Mr. Van, a prospective purchaser, responded to the advertisement, made an appointment to see the apartment, and was given a document called a 'property listing' in which an agency fee of 1 per cent of the sale price was stated to be payable. However, the document did not say who was to pay the fee. When Mr. Van asked who should pay the commission, he was told that the purchaser 'usually pays'.

Mr. Van entered into a contract for the purchase of the apartment at HK$3 million, and the estate agent claimed its 1 per cent of the purchase price, HK$30,000. Mr. Van then discovered that the Mr. Chung family had already paid 0.5 per cent to Richard Ellis and refused to pay the HK$30,000 to the estate agent.

Please advise Mr. Van his liabilities.