Slater became a free agent after opting out of his minor-league contract with the Rays on Tuesday, Robert Murray of FanSided.com reports.
Slater was slashing .305/.407/.453 in 35 games with Triple-A Durham, but since a promotion to the majors wasn't imminent, he'll seek a gig elsewhere. The 33-year-old outfielder has slashed .231/.311/.277 across 28 contests between three different big-league teams this season.