DUBAI/LONDON (Reuters) - Global banks who worked on Saudi Aramco's 2222.SE record initial public offering are pushing for an additional "incentive fee", three sources familiar with the matter said, as they try to boost relatively low earnings from the deal.

FILE PHOTO: A sign of Saudi Aramco's initial public offering (IPO) is seen before the start of a news conference by the state oil company at the Plaza Conference Center in Dhahran, Saudi Arabia November 3, 2019. REUTERS/Hamad I Mohammed

Aramco said on Sunday it had exercised a “greenshoe option” to sell an additional 450 million shares, raising the size of its IPO to $29.4 billion - comfortably the world’s biggest listing to date.

But the energy giant’s earlier decision not to market the deal internationally means most banks involved in selling Aramco’s shares will earn less than $5 million each, according to two of the sources, a low amount for such a large deal.

Now banks are in discussions with Aramco for the payment of the “incentive fee” under a clause in the contract that gives the Saudi government the discretion to pay banks an extra sum if the listing went well.

One source said they are arguing this will keep the banks committed if Aramco opts to sell more shares on an international exchange.

The payment of incentive fees - used to keep the base fees on a listing low and push banks to work harder - is increasingly common for IPOs in Europe and the U.S. but rare in the Gulf. The sources did not say how much extra the banks could be paid, although it would likely vary between institutions.

Aramco was not immediately available to comment.

The oil giant picked nine banks as joint global coordinators in September: JPMorgan JPM.N, Morgan Stanley MS.N, Bank of America Merrill Lynch, Citigroup C.N, Credit Suisse CSGN.S, Goldman Sachs GS.N, HSBC HSBA.L, NCB Capital and Samba Financial Group 1090.SE.

All the banks either declined to comment or did not immediately respond to a request for comment when contacted by Reuters.

The IPO was scaled back in size and scope before launch - with the targeted valuation revised down to $1.7 trillion from $2 trillion and the float cut to 1.7% of the company from the expected 2%.

Aramco also canceled marketing events in developed markets like the United States and Britain after lukewarm interest from foreign institutional investors who felt Aramco was not worth more than $1.5 trillion, sources said.

The decision meant the coordinating banks could only work on reverse inquiry and could not market or solicit investors directly.

“All protocols were changed and three banks were in control of the process,” said one of the sources, referring to NCB, Samba and HSBC.

The oil giant paid just 25 basis points as a base fee on its deal size, earning banks just over $60 million. In contrast, the IPO of China's Alibaba BABA.N in New York, which was the previous largest listing, earned banks $300 million.

The bulk of the base fee from Aramco’s IPO went to the three leading banks, said the sources, one of whom added that each got around $10 million.