A property agent who misled a married couple into handing her a $316,000 cheque to demonstrate interest in a Balmoral Road apartment, and failed to disclose she was the owner, has lost her lawsuit to claim the money.

A High Court judge found Ms Ong Keh Choo, who has 35 years' experience, to be "untrustworthy" and "opportunistic".

Ms Ong had sued American research scientist Paul Huntington Bernando and his wife, Vietnamese medical concierge Tran Hong Hanh, for cancelling the cheque after they found out about her non-disclosure.

Her case was dismissed by Justice Choo Han Teck in a written judgment released on Tuesday.

The judge said Ms Ong had misled Ms Hanh into thinking she and her husband were complying with a normal process for the sale and purchase of a property in which the cheque was only "for show".

Ms Ong has since sold the apartment to another buyer for $3.82 million.

The dispute between Ms Ong and the couple, who are Singapore permanent residents, arose on Oct 6, 2017, when Ms Hanh responded to her advertisement for the sale of the flat. The couple viewed the flat the next day.

According to Ms Ong, the couple were so eager to buy the apartment that they initially issued a cheque for the full purchase price of $3.16 million. She said she advised them to reduce it to 10 per cent in exchange for an option to purchase.

Her case was that the couple knowingly entered into the option but were trying to get out of the contract after experiencing buyer's remorse.

On the other hand, the couple said Ms Ong told them to issue a cheque "for show" and assured them she would not hand it to the owner. Later that day, Ms Ong met Ms Hanh alone and told her to sign a document.

Ms Hanh said she did not know she was signing an option to purchase the property. She later showed that document to a lawyer and was told it had "highly unusual" terms.

First, the option fee was 10 per cent of the purchase price - the value of the cheque - when the usual market practice was 1 per cent. Second, the couple had to pay the remaining 90 per cent upon exercise of the option, rather than the typical practice of paying upon completion. Third, the full sum was to be paid immediately to the owner rather than held by stakeholders.

Ms Hanh asked Ms Ong to destroy the cheque but the latter assured her there was nothing unusual about it. It was only after Mr Bernardo filed a complaint with the Council of Estate Agents on Oct 8, 2017 that the couple discovered Ms Ong was the owner, and cancelled the cheque.

Justice Choo accepted Ms Hanh's version of events.

As for Ms Ong, the judge said her failure to disclose her ownership of the property was a contravention of ethical guidelines.