FS1 NFL insider Mike Garafolo has confirmed the deal, which he says can't be finalized until the start of the NFL's new year Wednesday.
The Eagles signed Maxwell, a cornerback, to a six-year, $63 million contract a year ago. Maxwell, 28, played his first four seasons with the Seahawks, becoming a starter in his final year in Seattle before cashing in with Philadelphia. He started all 14 games he played in for the Eagles last season and intercepted two passes.
Alonso, a 25-year-old inside linebacker, came to the Eagles in a blockbuster March trade with the Bills for running back LeSean McCoy. He was one of the league's top young linebackers before a knee injury caused him to miss the 2014 season, but he played in only 11 games in 2015 and wasn't the defensive centerpiece the Eagles thought they were getting.
Garafolo reports that the Eagles will have to pick up a portion of Maxwell's salary and that the deal is contingent upon physicals, which is important because of Alonso's knee.
Compensation hasn't yet been determined. The Dolphins own the eighth overall pick in the April draft.