The move came after Buffalo already began shuffling its roster by returning rookie centre Mikhail Grigorenko to his team in the Quebec Major Junior Hockey League.
Brennan had one goal in 10 games this season. He also had a goal in 11 games with Buffalo last season. Selected in the second round of the 2007 draft, Brennan spent much of his first three seasons in the Sabres farm system.
The Sabres were forced to carry Brennan on their roster, because the team risked losing him for nothing by placing him on waivers.
The Panthers gave up a draft pick they acquired in a trade with Los Angeles.