Pacquiao lost a highly controversial decision to Timothy Bradley on June 9 in a bout that most felt Pacquiao won handily. Pacquiao initially said he would exercise the rematch clause, but ultimately opted instead to go for a fourth fight against Marquez.
"We are hopeful we can work out deals with both guys and come to terms for a fight on Dec. 8," Arum told Yahoo! Sports on Monday, but insisted contract terms hadn't been reached. "If they had been, I would tell you."
Earlier Monday, Arum said no decision had been made and said that Pacquiao was planning to fly to the U.S. on Thursday to meet with him to discuss his plans.
But they opted to focus on making a deal with Marquez later in the day.
Pacquiao and Marquez have a rich history, having fought in 2004, 2008 and 2011. They fought to a memorable draw at the MGM Grand in 2004, a bout in which Pacquiao knocked Marquez down three times in the first round.
In 2008, Pacquiao won a split decision over Marquez, then took a majority decision over Marquez in 2011 at the MGM in a bout that many felt Marquez deserved to win.
After that fight, Marquez said he wouldn't fight again in Las Vegas. Arum, though, said the venue has not been an issue in talks.
"He's never brought it up, not once," Arum said. "It's no issue, as far as I'm concerned."
The reason that Arum and Pacquiao decided to try to finalize a deal with Marquez rather than Bradley is unclear, but it likely has to do with pay-per-view sales.
Pacquiao-Marquez III did just under 1.3 million sales, while Pacquiao-Bradley did slightly over 900,000.
In addition, after the outrage over the decision of judges Duane Ford and C.J. Ross, who had it 115-113, or seven rounds to five, for Bradley, many fans said they wouldn't buy a rematch. They took to social media in massive numbers to express their outrage.
Arum didn't say what influenced the decision toward Marquez and away from Bradley. Pacquiao manager Michael Koncz denied that Marquez had been chosen, but declined to speak further on the matter.
"We're still looking at both guys and we haven't decided to go with Marquez," Koncz said. "We just have been talking about both guys and today we happened to discuss a purse for Marquez. But I don't want to get into it and start to negotiate or divulge what we're thinking in the media.
"Once we have a deal and have signed contracts, I'll be happy to answer those questions."