D. Douglass wanted to prepare the reader for the effect the conversation had on his own emancipation.
The sentence describes how the slave's conversation with his master resulted in the voluntary emancipation of the slave, indicating that conversations between slaves and their masters could sometimes have unexpected and positive outcomes. This foreshadows the effect that conversations would have on Douglass's own emancipation, which was also the result of unexpected circumstances and conversations. Therefore, option D is the best description of why Douglass used this sentence in his narrative.