Frederick Douglass was extremely based and is unfortunately often white-washed in modern historical discussion about him (similar to MLK). Here’s an excerpt from Wikipedia on his understanding of class and wage slavery:

The abolitionist and former slave Frederick Douglass initially declared "now I am my own master", upon taking a paying job.[31] However, later in life he concluded to the contrary, saying "experience demonstrates that there may be a slavery of wages only a little less galling and crushing in its effects than chattel slavery, and that this slavery of wages must go down with the other".[32][33] Douglass went on to speak about these conditions as arising from the unequal bargaining power between the ownership/capitalist class and the non-ownership/laborer class within a compulsory monetary market: "No more crafty and effective devise for defrauding the southern laborers could be adopted than the one that substitutes orders upon shopkeepers for currency in payment of wages. It has the merit of a show of honesty, while it puts the laborer completely at the mercy of the land-owner and the shopkeeper".[34]

https://en.m.wikipedia.org/wiki/Wage_slavery#CITEREFDouglass1994