Long Road to Freedom - Kate Messner

nRanger is a time-traveling golden retriever with search-and-rescue training. In this adventure, he goes to a Maryland plantation during the days of American slavery, where he meets a young girl named Sarah. When she learns that the owner has plans to sell her little brother, Jesse, to a plantation in the Deep South, it means they could be separated forever.

Sarah takes their future into her own hands and decides there's only one way to run -- north.