I ran into that too and realized it was because of the new "employee girls". I think the shift was to create more of a hierarchy in the house.0.7.10.0 : the usual way to send slaves to work, doesn't work for me now. the button simply doesn't show up.
the save is from 0.7.9
neither at the bar or auction house. tried new slaves and new game.
I have employees new action panel, but slaves, don't have it.
I've tried with 0.7.9, and shows no problems, with the same file
-Cow/catgirls
-Slaves in the cages which get trained and can be made to clean/cook in the house.
-Favored slave on the mat in the bedroom.
-Employee girls who can work outside the home.
-Servants to take care of the home and other slaves/petgirls.
-Mistress