HONG KONG (BLOOMBERG) - Alibaba's chairman Jack Ma and vice-chairman Joseph Tsai will bolster the company's share buyback as they agreed to spend US$500 million (S$703 million) of their own money purchasing stock in a vote of confidence for the e-commerce operator.

The billionaire co-founder and his lieutenant will bankroll the purchase of shares currently trading at US$66.91, Alibaba said in an e-mailed statement on Monday.

The company announced in August Ma's and Tsai's intention to contribute to a US$4 billion buyback without specifying how much they would spend.

Alibaba is trading below its initial public offering price of US$68 after plunging 20 per cent in the past year as it grapples with slowing growth, the result of its reliance on a decelerating Chinese economy.

It's getting squeezed by price cuts and competition in bigger cities, while facing lawsuits amid challenges in dealing with counterfeits.

As China's largest online emporium, Alibaba is the standard-bearer for an economy that's navigating a bumpy transition from a high-speed export economy to one anchored by consumer spending.

The broader deceleration and Alibaba's own scale - it handles more e-commerce than Amazon.com Inc. and EBay Inc. combined - curtails its pace of expansion.

The company said last August it would buy back shares over a two-year period, mainly to offset dilutions such as from employee compensation programmes.

Ma and Tsai have seen their personal wealth eroded by the share price plunge.

The co-founder is currently worth US$26.8 billion, making him China's richest person according to the Bloomberg Billionaires Index. His deputy is worth about US$4.4 billion.