If you have time, I would recommend to you to read the extensive research report done by Monticello about the whole question of Sally Hemmings. Jefferson kept extremely detailed dairies about the running of the plantation so that we know exactly when he was in Virginia. In those notes are dates when each of Sally’s children were born, which allows the possibility of tracing back to conception. Mr. Thomas Jefferson was the only Jefferson male present for each of those times. There is a lot of other information as well.
What I was stunned to learn is that Sally Hemmings was Jefferson’s wife half sister! Her mother was the concubine of her father and that had been the situation in that slave family for a couple generations so that Sally was actually mostly of Caucasian ancestry. But the laws of Virginia forbid her to be listed as such because her mother was a slave. So contrary to TV presentations, Sally did not have the appearance of being a slave. When she was freed and lived in her own home after Jefferson’s death, the census listed her as white - as were several of her sons.
Sally Hemings was never “freed”. She was not given her freedom in Jefferson’s will as were two of her sons. Martha Randolph (Jefferson’s daughter) put Sally “on her own time”, meaning she essentially retired her from working. Late in 1826 Sally was allowed to leave Monticello without being pursued. That meant that she could leave and no effort to would be made to return her to Monticello. This is how two of her daughters were allowed to leave Monticello some years earlier. Sally moved in with two of her sons, that lived near Charlottesville. She would remain there until her death in 1835