I don’t think it’s wrong to speculate, but this misses the point of enslaved humans, or the project of forced labor. The purpose is to extract the labor for concentrated economic benefit. Slaves weren’t free either and cost between $12,000-$45,000 depending on the market conditions and the purpose of the enslaved person. Typically, they were two to three times the value of the land and were considered taxable. During the cotton period, international investors in Western Europe and in the US in financial centers sunk money into the buying and use of slaves due to the insatiable demand for cotton and rapid acquisition of the land at government auction. In 1860, tax records from the state of Georgia indicate that the property of slaves accounted from 42% of the state budget while land was 21% of the 13 or so categories of taxation. So “gifting” had expansive consequences both on the front end of buying, the calculation of extraction of labor, and the registration and taxation afterwards. This gifting does not appear to be a widespread practice, and rather rare since children would be angry at their inheritance being given away. There were a few “fancy girls” given between ultra rich, but only when a new owner could bear the financial consequences. Also, caring for a slave was expensive relative to usage, and the support of the slave—food, clothing and shelter—came from the economic output of the person.
I would say it’s not really a close comparison at all. Pets are an economic burden, a joy, and live with black Americans, white Americans, Asian Americas—across all ethnic lines in the US and Europe. They provide joy if they aren’t also hunting dogs, and not a consideration of keeping. I don’t think this comparison will “get you in trouble” but rather shows how the depth of the economic calculation of slaves has ostensibly been “vanished” from our collective understanding of this institution, allowing for all kinds of comparisons that are not accurate (“slave labor,” “slave wages” in describing work, etc).