The Senators receive defenseman Nick Holden -- a 10-year veteran who has played for five teams -- and a 2022 third-round draft pick.
Dadonov carries a $5 million cap hit for the next two seasons. Fleury had a $7 million hit.
Dadonov, 32, scored more than 20 goals three straight seasons with the Panthers from 2017-20. He had a career-high 70 points in 2018-19. After signing as a free agent with the Sens, he dipped to 13 goals and 20 points in 55 games last season.