From Halopedia, the Halo wiki
Sacrifice is a short story that is included as additional content in the novel Halo: Shadows of Reach in limited-edition exclusive copies sold by Wal-Mart, in stores or online. The short story is fully self-contained, though connected to the Shadows of Reach novel in a "unique way". The story is uncredited to a specific author, only credited within Shadows of Reach to 343 Industries.
The short story was not written by Troy Denning, but by the people at 343 Industries. It was written when Troy Denning was revising Halo: Shadows of Reach as 343 Industries saw a chance to expand on a detail in the book to tie-in and prepare for something else.