The term enlisted man's sack coat is often seen, especially in connection with the outfit Grant wore at Appomattox, but a misnomer nevertheless. Like those for enlisted men, Federal officers' Regulation uniform consisted of a nine-button single-breasted frock coat for captains and lieutenants or one of several different double-breasted versions for majors and above. However, officers were authorized to wear a fatigue jacket or sack coat in various orders of undress. Officer's uniforms, like their sidearms, horses, and virtually everything else, were NOT "issue items" - they were required to BUY their own. Therefore, officers did not wear anything actually intended for enlisted men but BOUGHT versions that usually differed in details of tailoring, color, cut, etc. (This is not to say a high-ranking officer might not go down to the commissary and grab something in a pinch, but it wasn't supposed to happen.) The sack coats or fatigue jackets that were quite often worn by the brass were almost always private-purchase items similar in cut and color to those issued to their men but usually differing in detail.
Here's an example from my collection of a very old first lieutenant wearing what is obviously a private-purchase four-button sack coat that in addition to being cut very full to accommodate his girth also has an outside slash breast pocket. Other "custom" features might include cuff buttons and the presence or absence of lining.
I can’t say it better than James N., but he is correct, General Grant’s surrender sack coat is in the Ray Ritchie collection and is most certainly a private purchase sack coat. I will add that lower ranks did use issue clothing, particularly when they were promoted from the ranks. I have a pair of issue trousers that were tailored to be officer’s pants. There is a thread somewhere here on the pair. A newly promoted 3rd Lt. May not have access to private purchase wear particularly when in the field. The newly minted Lt. may not have the means to purchase a new kit immediately.