"We are still working on the details," he told Anderson. "They showed the most interest. They are a playoff team and I think they have a lot of great talent."
Also, defensive tackle Vernon Butler, 25, has agreed to terms with Buffalo, a source told Anderson.
The additions of Jefferson and Butler come after Buffalo lost defensive tackle Jordan Phillips to the Arizona Cardinals in the early stages of free agency. Phillips led the team with 9.5 sacks in 2019.
Phillips' departure left an opening behind 2019 first-round pick Ed Oliver, Star Lotulelei and Harrison Phillips. Jefferson should split reps at three-technique defensive tackle but could also be used at defensive end, where the Bills are also relatively thin.
Jefferson started 24 games over the past two seasons for the Seattle Seahawks while playing on the edge and in the interior of their defensive line. While he's listed in some places as an end, most of his snaps over the past two seasons have come at defensive tackle.
He recorded a career-best 3.5 sacks last season, the second most on the team, after a three-sack season in 2018. Overall, Jefferson has 7.5 career sacks in 39 games since the Seahawks traded up to draft him in the fifth round in 2016 out of Maryland.
He tore his ACL as a rookie and rejoined the Seahawks in 2017 after a brief stint on the Los Angeles Rams' practice squad.
Jefferson, who turns 27 on March 31, made $2.025 million last season while playing on an original-round RFA tender.
In 2017, Jefferson was fined for attempting to climb into the stands after Jacksonville Jaguars fans had thrown drinks at him and allegedly said things about his family as he was leaving the field following an ejection.
ESPN's Brady Henderson contributed to this report.