The Bills improved the talent along their defensive line Tuesday night.
Butler will get a two-year contract for roughly $16 million that can be worth up to $18 million, according to Rapoport. Of that, $9.3 million is guaranteed.
Jefferson will sign a two-year deal, though terms are not known yet.
Butler, 25, spent his first four seasons in Carolina after the Panthers made him the 30th overall choice in 2016. He appeared in 52 games with nine starts.
Butler leaves after making 77 tackles, eight sacks and three forced fumbles.
Jefferson, who turns 27 later this month, was a fifth-round choice of the Seahawks in 2016. He played 39 games with 24 starts for Seattle and made 59 tackles and 7.5 sacks.