Mark Twain said that the slaves were past masters of fooling their owners. Plautuis has nothing on Mark Twain.
Julia Grant she had a great deal more enjoyment down on her father's plantation when she was with her fathers slaves. But in the north there was no fun, just work work work.
She tells, for example of the women who did the sewing work. they would come 1/2 day per week, silently, and get the job done, expensively, that would require four slaves to do over the course of a week with Julia sitting in and participating in the chat circle.
Grant himself liked plantation life where work was done in a slowly and not very well. Grant didn't like working for his father's store, and he wasn't a great employee.
When leaving for Galena in 1860 Julia states her father advised her not to take the four "servants" he had "given" to her. He said this, according to Julia, because he thought she may not like the north and may return home fairly quickly and of course her "servants" would become free and stated that he did not think she could do without "servants".
In Galena the Grant's had one maid/cook "Jennie" who according to Julia
"did all of our [house]work." Despite her adjustment to paid labor Julia claimed she was pleased
"at the ease with which my housekeeping moved on."
Grant would write to his wife on the outbreak of the war in 1861 that Missouri would be better off as a free state...
"Missouri will be a great state ultimately but she is set back now for years. It will end in more rapid advancement however for she will be left a free state. Negroes are stampeding…and those who do not will be carried…South so that the destiny of the state, in that respect, may now be considered settled by fate and not political parties."
–USG to Julia Grant 5/1/61
He said the end of slavery would be a natural consequence of the rebellion because...
“…negroes will depreciate so rapidly in value that no body will want to own them…"
-USG to his father Jesse Grant 5/6/61
Julia Grant writes in her memoirs that by early 1864
"Our colored people [in St. Louis] had all left, but their places were readily filled by German and French men and women, who were most excellent substitutes." Although she would state that the absence of their old family servants was missed when they were entertaining many guests in the summer of 1864.
The subject of Grant's aptitude for different occupations, including the Galena leathergoods store, is one of contradictory source accounts and debate. One thing is sure, he fancied himself a farmer and never fully gave up the hope of farming and working with horses. After the Civil War he tried to secure a farm property near Washington DC, but it ended up falling through. He felt much more at ease with his hands in the dirt and taking care of horses than he ever did in the political world of the city. This is a testament to his willingness to put his own desires aside to put himself in uncomfortable positions for the sake of his country.
I wasn't sure what to take out of the OP, so thanks for clarifying. I'm not surprised Julia did not understand this on a certain level as she had not been exposed to a 'free labor' market (as opposed to a 'slave labor' market). If we take as a bottom line that we can only go by our own lived experience, I'm assuming Julia's early life experiences did not lend her to understanding this easily. That's not to say the understanding didn't come.
I think it's important to note that Julia would have been well aware that her husband was employing free blacks on the farm. Missouri was a mixed free/slave labor market.
In 1880 a Canadian-born abolitionist, Dr. Alexander M. Ross who had worked to free slaves in the south before the war, wrote of his experiences and sent the book to the Grant's. Grant wrote back that he had not read the book but that Julia read it and apparently the authors description of slavery had a powerful effect on her. Grant stated in the letter...
"...although she [Julia] had been raised in a slave state, and always owned slaves – as her father did – while slavery existed in our country, she said she could not see how it was possible that any body ever justified such an institution."
-USG to Dr. Ross 9/11/1880
Despite this admission Julia still relates her childhood experiences with slavery through rose colored glasses in her memoirs, though that was her tendency to put a positive spin on negative or controversial subjects.
Both Grant and Julia's views on slavery and racial equality certainly evolved throughout their lives as it did for many Americans of the era.