I believe the Robert Sellers released on February 27, 1864 at Knoxville to be your GGrandfather. The intelligence recording his release, Lewis' Artillery is recorded in the Unfiled papers which indicates that the CSR compilers could not identify a battery named Lewis. I believe this resulted from a typographical error in transposing documents, Levi's being rather similar to Lewis' with the 'W' substituting for the 'V', see:
LEVI's
LEWIS
On the date Robert Sellers deserted, September 2, 1863, the Legion was operating in the area of East Tennessee and North Carolina. Hence, his apprehension by Federal forces and subsequent release in Knoxville, then under Federal control, would make sense.
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Robert Sellers, age 39, Private, (2nd) Company D, Walker's Battalion, Thomas Legion, North Carolina Troops, enlisted February 9, 1863 at Strawberry Plains, Tenn., absent sick, at home 3 miles from Strawberry Plains from March 15, 1863 through June, 1863, subsequently transferred to Levi's Battery**, Thomas' Legion August 1, 1863, present on the July & August, 1863 muster roll, deserted September 2, 1863 as recorded on the Company's Sept/Oct, 1863 muster roll, no further records
M270: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of North Carolina
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Robert Sellers, Private, Lewis Battery, is recorded on a List of Rebel Deserters, released on taking the oath of allegiance at the office, Knoxville, of the Provost Marshal General of East Tennessee, during the month ending February 29, 1864, released February 27, 1864 [Note: This is the one and only card under his name]
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Robt. D. Sellers, Rebel Army, Reference Card. See Manuscript No. 404***, Page ______, dated Sept. 29, 1863 [Note: This is the one and only card under his name]
M347: Unfiled Papers and Slips Belonging to Confederate Compiled Service Records
*** This document most likely resides deep in the bowels of the National Archives in Washington, DC. I would suspect, based on the date, it most likely relates to your Gramps, if the middle initial matches your records.
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** The Legion's " . . . artillery battery, John T. Levi's Light Artillery Battery (a.k.a. Louisiana Tigers), formerly served in the Virginia State Line Artillery and was added to the legion on April 1, 1863."
June 20, 1863, at the Battle of Strawberry Plains Bridge, Tenn., the Union army captured Levi's four pieces of artillery and eleven of the battery's men.
"In the fall of 1863, Colonel Thomas ordered . . . [6 Company's] . . to enter Cherokee County and press into service absentees and furloughed men of his and other commands. When the Confederate forces in the Cumberland Gap capitulated on September 9, 1863, Colonel Thomas and the two Cherokee Companies departed Strawberry Plains for Western North Carolina."
http://thomaslegion.net/