He has one season remaining on a three-year, $18.75 million contract ($6.25 million average annual value) he signed with the Maple Leafs on July 2, 2017.
"There were a few teams in the mix," Toronto general manager Kyle Dubas said. "We couldn't take any salary back and wanted to protect ourselves, and we didn't want to give up any of our younger players. In fact, we were trying to get them all signed.
"In our situation with our cap, we didn't want to do one of those trades where it was moving salary or a cap hit that was going to take somebody off our roster, so you start looking at your draft capital. When you start talking about a $6.25 million cap hit and $4 million in cash, you have to pay a hefty price and that's what we've done that."
Carolina general manager Don Waddell said the trade was not made with the intent to buy out Marleau.
"No, we'll talk to him," he said. "...The plan would be to meet with Patrick and talk to him to see where he's at, and if he wanted to be a Hurricane, we'd certainly love to have him as a Hurricane."
Selected by the San Jose Sharks with the No. 2 pick in the 1997 NHL Draft, Marleau has 1,166 points (551 goals, 615 assists) in 1,657 regular-season games and 127 points (72 goals, 55 assists) in 191 playoff games.
He ranks fourth among active players in points, second in goals, and fifth in assists. He is first among active players in playoff goals.
"It's a shot in the dark," Hurricanes coach Rod Brind'Amour said. "I think both teams have a plan here with what they wanted to do. If we can get him to play, it's a bonus, but we understand what the deal was.
"It's a shot and if it doesn't work out, that's not the big part of the deal."