Arizona will only pay Willis the minimum salary of a prorated $400,000, with the Tigers paying the remainder.
The Tigers acquired Willis, a two-time All-Star and a former 20-game winner, as part of the 2007 deal that included Miguel Cabrera.
Willis is 1-2 with 33 strikeouts, 29 walks a 4.99 ERA in nine starts. He won three games in three seasons in Detroit, appearing in 24 games.
Buckner, who was 0-3 with an 11.08 ERA in three starts this season.