The slaves that served Julia Grant were freed when the state of Missouri outlawed slavery. They were never her property. They belonged to her father. He would not allow the slaves to accompany Julia when she traveled with her husband out of state of Missouri.
Constitutionally he could not. It took the 13th Amendment to do that.
As to when Grant sold his slave you are right except that his wife kept hers slaves until they escaped.
Julia Grant did not own those slaves. Her father did. Julia Grant had use of them but did not have the power to free them even if she had been so inclined.
I indeed was mistaken about when Grant freed his slave. Don't know where I got that but should have verified it before posting!!!
Been there, done that.