Duncan McMillan , Private, Company E 15th Confederate Cavalry, enlisted 15 November 1862 at Camp Lomax, Florida. He is identified as being present on the musters for November and December 1863, March and April 1864, and May and June 1864. The only other document on the film is an index card for a "receipt for clothing for the third quarter of 1864" which is dated 2 September 1864. There is no mention of a resignation on the film. I hope this helps.
If you need a copy of this, let me know.
Regards,
Jim