Advisers on the potential 50 billion pounds merger of Glencore and Xstrata are facing a reduced fee pot of up to $140 million (88 million pound), based on estimates from Thomsonreuters/Freeman Consulting.
Glencore Chief Executive Ivan Glasenberg has recruited Citigroup and Morgan Stanley, people familiar with the matter said, which will share $50 million to $70 million in the event of a successful deal.
The two firms are long-standing advisers to Glencore, having also worked on its $10 billion listing last May.
Credit Suisse, also a top bank on the IPO, has not yet secured a role in the situation, the people said.
Xstrata chief executive Mick Davis has called on Nomura and Goldman Sachs alongside JP Morgan and Deutsche, the people said.
These four banks also stand to earn $50 million to $70 million if a deal goes through.
Fees for both teams are about 20 to 30 percent lower than normal for a deal of this size, Freeman Consulting said, because Glencore's exising 34 percent stake in Xstrata and close ties between the companies means less work for advisers.
Failure to clinch a deal will see both teams earn just 10 percent of the fees available.
Xstrata and Glencore are in talks over an all-share merger that could shake up the mining industry with its biggest deal to date.
While their two groups have held on-off talks over years, speculation over a tie-up accelerated with Glencore's bumper listing, which handed Glasenberg the currency for deals.
All of the banks and the two companies declined to comment.
(Additional reporting by Clara Ferreira-Marques; Editing by David Cowell)