So by your definition any position short of economic independece would be slavery?
I'm not depreciating suffering. Calling employees in a poor bargaining position "slaves" is muddled argumentation. Any arbitrary non-negotiable position is not slavery - unless one wants to be poetic (i.e. "we are all slaves to laws of physics" etc).
As to the definition of slavery I would refer to wikipedia: https://en.wikipedia.org/wiki/Slavery
"Slavery is a legal or economic system in which principles of property law can apply to humans so that people can be treated as property,[1] and can be owned, bought and sold accordingly, and cannot withdraw unilaterally from the arrangement."
Capital has been always a lopsided thing - few have it. It's quite different not having capital, than being treated as a capital asset and having almost no human rights.
As a fairly modern example Frederick Douglas (https://en.wikipedia.org/wiki/Frederick_Douglass) has more than a few words to say about his time as a slave.
"I do not remember to have ever met a slave who could tell of his birthday... A want of information concerning my own was a source of unhappiness to me even during childhood. ... I was not allowed to make any inquiries of my master concerning it. He deemed all such inquiries on the part of a slave improper and impertinent,"
http://www.gutenberg.org/files/23/23-h/23-h.htm
Being bound to only few means of sustenance is one thing. Having ones birthday robbed is another entirely.