This logic deprived me of some IQ points.
I could understand if she was an indentured servant of some kind selling herself for a defined period of time while keeping her rights, somewhat similarly to "The dubai girls" (some of the accounts are very gross so beware before you look that up) , but as a slave thats not the case. As a slave the master has not obligation to reward you for your work. This fortune she's talking about will never manifest and the quality of her living conditions aren't guaranteed either.
In a world where you could make a cleverly worded craigslist post and have someone at your door ready to be degraded in any which way for enough money, the likely hood that you'd find a decent person who is looking to buy slave in extremely slim. In fact id go so far as to say the vast majority of them are malicious.
I suppose considering this is a manga I should be suspending my disbelief a bit more, but this shit is just stupid.