Thanks for your very informative post. To the best of my knowledge, slaves were never promised emancipation in return for military service. Section 5 of the act to enroll slaves passed by Congress on March 13, 1865, reads as follows,

That nothing in this act shall be construed to authorize a change in the relation which the said slaves shall bear toward their owners, except by consent of the owners and of the State in which they may reside, and in pursuance of the laws thereof.

Service files of Texans serving with Forrest's original battalion will be found in M268, Compiled Service Records of Confederate Soldiers Who Served in Organizations From the State of Tennessee, reels 11-14, 3rd (Forrest's) Cavalry. That is the source for much of the information posted earlier.