Does anyone know when a slave of a Union man (western KY, western TN, etc.) was granted his freedom if he enlisted into a U.S.C. Regiment?

There was a remark by someone below (can't find it now) about slaves enlisting and being given their freedom. This implies that they were given their freedom upon enlistment. That would be true of slaves belonging to Southern sympathizers or Confederates, but did the same apply to slaves belonging to Union owners?

Did they have to serve a specific period of time? Did they need permission to enlist from their Union owner?
Thanks,
Ann