The only difference as compared with the old, outspoken slavery is this, that the worker of today seems to be free because he is not sold once for all, but piecemeal by the day, the week, the year, and because no one owner sells him to another, but he is forced to sell himself in this way instead, being the slave of no particular person, but of the whole property-holding class. For him the matter is unchanged at bottom, and if this semblance of liberty necessarily gives him some real freedom on the one hand, it entails on the other the disadvantage that no one guarantees him a subsistence, he is in danger of being repudiated at any moment by his master, the bourgeoisie, and left to die of starvation, if the bourgeoisie ceases to have an interest in his employment

-Engels

https://www.marxists.org/archive/marx/works/1845/condition-working-class/ch05.htm

The slave is sold once and for all; the proletarian must sell himself daily and hourly.

The individual slave, property of one master, is assured an existence, however miserable it may be, because of the master’s interest. The individual proletarian, property as it were of the entire bourgeois class which buys his labor only when someone has need of it, has no secure existence. This existence is assured only to the class as a whole.

-Engels, again

https://www.marxists.org/archive/marx/works/1847/11/prin-com.htm