Goldman Sachs Group and Morgan Stanley asked some counterparties to cancel or amend trades in Singapore-listed Jardine Matheson Holdings, after a US$41 billion (S$56 billion) flash crash on Thursday saw shares change hands far below market level, according to people familiar with the matter.

Some of the amendment requests were to settle the trades at a higher price, according to the people, who asked not to be named. Goldman and Morgan Stanley declined to comment on what triggered the brief plunge.

Speculation has been swirling about Jardine's dramatic start to Thursday's session, when some 167,500 shares changed hands at US$10.99, compared with the previous day's close of US$66.47, then bounced back within minutes.

That implies a loss of about US$9 million for those who sold at the pre-market level, and an instant windfall for those on the other side of the trade, according to calculations.

The Singapore Exchange decided not to cancel the trades after reviewing the incident, as sellers had "ample time" to withdraw their orders if they did not want to offload shares at the low price.

The exchange attributed the decline to sell orders that overwhelmed bids during the pre-open, for which neither a "fat finger" error nor a malfunctioning computer system was responsible.

BLOOMBERG