Which is a very narrow period and a small number of individuals involved in a macro/global historical view.
I don't think the slaves that build the pyramids, were taken from the Slavic countries, or were captured by rival tribes the world over ever thought they were promised any wages.
The overwhelming majority of slaves throughout history were slaves by conquest, no coercion or enticement.
The majority of the slaves in the time period you're referencing became such in that way as well. There were some folks on the indentured servitude side of spectrum that devolved into outright slavery through deceit, but that's hardly the majority.