Arum presented "what he thought was the best proposal he can bring" during a two-hour breakfast meeting with Pacquiao in Manila, said Michael Koncz, an adviser of the Philippine boxing idol.
"Manny has some additional requirements, requests, which Arum didn't think was a problem," Koncz said. "The requests of Manny were so realistic that Arum doesn't feel it's a problem and it's pretty much a done deal."
He said the contract still needs "fine tuning," declining to elaborate.
"We all believe that it will be done," he said.
In an interview with GMA television, Pacquiao said, "March 13 is OK."
"The difference between Floyd and others I have fought is that Floyd makes a lot of trash talk that should not be imitated by young people," he said.
Arum could not be reached for comment.
Koncz, who is in charge of Pacquiao's boxing promotion company in Nevada, said Pacquiao is "very comfortable" with the date.
Pacquiao is running for a congressional seat in May 2010 elections and official campaigning won't start before April.
If the bout goes ahead, it will match two men regarded as the best pound-for-pound fighters.
Boxing fans have been looking forward to a super fight between Pacquiao and the undefeated Mayweather after the Filipino champion defeated Miguel Cotto last month.
The March 13 date comes four months after the Cotto fight _ a shorter period of time than the six months after Pacquiao defeated Ricky Hatton. But Koncz said it will be enough time for Pacquiao to get back in shape.
"Nobody knows his body better than Manny," he said. "If Manny feels that that's plenty of time to rest and recover, then you know, he has to do what he feels is right."
He said Pacquiao "cannot train for more than seven weeks."
If the fight is made, there would likely be a Jan. 11 press conference in New York to announce it. The bout could be the richest ever, assuming pay-per-view projections are accurate.
Representatives of the Superdome in New Orleans and the new Dallas Cowboys' stadium have expressed interest in hosting the fight, as has the MGM Grand hotel-casino in Las Vegas, where the fighters have fought their biggest fights. There is also a proposal for a 30,000-seat temporary stadium on the Las Vegas Strip across from the Wynn resort.
Oddsmakers are already taking bets on the fight, with most making Mayweather a slight favorite.
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