I have read, but cannot find where I read it, that Buckner expected generous terms from Grant when the former surrendered at Ft. Donelson. Buckner had known Grant at West Point, and moreover had settled Grant’s enormous bar tab in New York out of generosity, and so thought that he would be treated well and his army allowed to retreat. He was disappointed in this hope, and that’s where the “Unconditional Surrender” Grant nickname was born.
Grant smoothed Buckner’s ruffled feathers after the surrender, and even offered to loan him some cash in case he needed it. That was to repay what Buckner had done for him at the time of his resignation from the army many years before.
https://freerepublic.com/focus/f-chat/4038469/posts#17