Good point. I read your reply there, but thought it more relevant to reply here. I was grasping a bit to try and understand that one. It is such a small and early section for such a large, varied and recent development in the larger economy. There were some forms of slaves getting their freedom individually though, and it was even somewhat common for slaves in Saint Domingue to not only get their freedom and become either peasants, craftspeople or even land/slaveowners themselves. And in other colonies with greater numbers of slaves, Maroons also formed parallel societies to that of the coloniser. The USA is a bit of a weird one for slave societies because they had a way higher amount of indentured servants or otherwise white workers compared to most Atlantic colonies, so they could enforce their racist laws with more effect.
I guess what I meant there was that slaves and their associated labour are individual property subjected to the whims of their single owner, while that of the proletarian is collectively dominated by the entire capitalist class of a society with no formal sale of workers between proletarians needed. I'm not sure if Engels meant that as I've not read much from him specifically on slavery, that was just my crackpot interpretation and I'll trust you there.
Edit: posted by accident, fixed now.
Fair enough, I was confused about the differences between genzedong.xyz and genzedong.org before, but will try joining this weekend.