It was not until Dec 1875 that Capt Benteen became the senior captain of the 7th Cav. At the time the regiment was organized in 1866 the senior captain was William Thompson. In the reduction of the Army that occurred in 1869-1871, Capt Verling Hart was transferred from the infantry to the 7th Cav. Normally a voluntary transfer from one regiment to another would have to be handled as an exchange and officers transferred into a new regiment would lose their seniority. But the transfers that where due to the reduction of the army allowed officers to retain their seniority. Hart, whose date of rank preceded Thompson's became the 7th Cav's senior captain.
Hart eventually became the senior captain of the cavalry and in due course was promoted to major, at which point Thompson again became the senior captain. When Thompson retired in Dec 1875, Benteen became the senior captain.
During the Yellowstone Expedition in 1873, Hart was a wing commander and Custer's second in command.
As the returning Black Hills Expedition neared Ft Abraham Lincoln, scouts came in with dispatches and word that Capt Hart's child was sick at Ft Rice. As I recall, one source said that Hart was not allowed to leave the expedition until the command was a days march out of Ft Lincoln. Another source says that Hart was given permission immediately and that the day before the regiment arrived at Ft Lincoln Hart left with the scouts who had brought the news.
This is the nearest thing I have been able find that approaches the story that Custer denied leave to Capt Benteen for the same purpose.