When the Postal Savings Bank of China announced it had raised $7bn from a group of international investors it marked a significant step towards what is expected to be one of the biggest Chinese listings in 2016.

Alongside China Life and giants of global finance such as UBS and JPMorgan, it raised funds from the World Bank’s private sector arm, the International Finance Corporation, representing a turnround for a state bank that just three years earlier had seen its president charged with corruption.

But the deal has ​become​ emblematic of the often subtle ways in which Beijing and Washington battle for influence behind closed doors at the World Bank. It has also cast a shadow over the departure of China’s most senior official at the World Bank, former Goldman Sachs banker Jin-Yong Cai, who after a turbulent three years at the helm​ of the IFC leaves this week to return to Asia and the private sector.

While Mr Cai and other senior World Bank officials insist that he is leaving 10 months before the end of his four-year term for personal reasons and there are no allegations of any wrongdoing, his exit ​followed an unusual rebuke by the World Bank’s board for the Postal Savings Bank of China transaction that he championed. It comes amid what senior bank officials say were mounting concerns by the US and others over alleged favouritism involving a growing number of IFC deals with Chinese state-owned companies.

At $300m, the IFC’s investment paled in comparison with the $2.5bn China Life paid for its 5 per cent stake in the Postal Savings Bank. However, it was the largest the IFC had ever made in a Chinese bank and Mr Cai saw it as an example of the “big bang” deals for which he ​h​​oped​ to be remembered.

According to senior World Bank officials though, big shareholders questioned how the investment fitted with the IFC’s mandate to lend to private companies in places where capital was either unavailable or too expensive and other investment options were not available.

​Plenty of other ​investors were​ lining up for a stake in the Postal Savings Bank of China. Moreover, it was unclear, the officials said, just what the development impact — a crucial test for IFC investments — would be despite Mr Cai’s claims that it fit with the bank’s “financial inclusion” strategy.

“It is unclear to me what the role is for the IFC in [the Postal Savings Bank of China],” says Peter Woicke, a former JPMorgan banker who ran the IFC from 1999 to 2005 and oversaw its initial investments in China’s banking sector. “If that [investment returns] is why you are doing it you might as well be Goldman Sachs. But the job of the IFC is not just to be Goldman Sachs.”

The IFC and Mr Cai argue the investment in the Postal Savings Bank will help bring financial services to many of China’s “unbanked” and provide a healthy investment return as well.

T​he clash surfaced when the World Bank’s board considered the IFC’s $300m investment in June. When the 25-member board gathered to vote on it June 9, according to minutes, the representatives for the US, Japan and big European shareholders including the UK, Germany and France all abstained. Altogether nine directors responsible for 56.4 per cent of the IFC’s voting shares withheld their approval.

Because those directors abstained rather than voted against, the investment went ahead. But in the nuanced arena of World Bank board politics where outright votes against projects backed by powerful shareholders such as China are rare the vote amounted to a dramatic rebuke to Mr Cai.

“It’s very unusual,” said Chad Hobson, executive director of the Bank Information Center, a Washington-based campaign group focused on the World Bank. “They don’t [normally] go to the board when they are going to lose a vote.”

The vote, according to current senior bank officials, came after the US had for more than a year been expressing growing concerns about a number of IFC transactions involving Chinese state-owned companies around the world. Those concerns had seen the US either abstain or vote against a renewable energy project in Pakistan and a proposed $140m in investment in IFC projects involving Chinese state-owned grain trader COFCO.

It also came as Chinese stock markets were in turmoil and just ahead of the release in July by the World Bank of a report, which the bank later pulled, in which it warned that China’s “distorted” financial sector was in urgent need of reform.

Mr Cai defended the investment but noted that anything to do with China often had extra sensitivities attached.

“There are always different views on projects in China,” he said. “I understand that.”

With its 40,000 mostly rural branches, the Postal Savings Bank of China allowed the IFC to help spread financial services further into China’s countryside where some 235m still did not have bank accounts, Mr Cai said. It would also enable the IFC to learn from one of the world’s biggest postal banks. “How do we gain our experience? We can’t just get it by reading books. We have to be involved in transactions,” he told the Financial Times.

Mr Cai insisted it was a good investment for t​​he​ IFC, which​ has had to sell other equity holdings in China to fund dividend payments to the World Bank to finance its low-interest lending to the world’s poorest countries.

Some of the IFC’s other equity investments had had a difficult time, he said. In the year to June 2015 it​ took $700m in writedowns on its equity investments in emerging markets, according to figures provided by the IFC. Chin​a​ had been a rare bright spot, Mr Cai says, although the IFC declined to provide details of how its investments there had performed amid​ recent turmoil in the Chinese market.

“We have made so much money from our investments [in China]. We are so lucky. We thank the Chinese for giving us this opportunity,” Mr Cai said.