What is the difference between causing someone a loss of $, and causing someone a loss of wages by testifying that they are a slave? R’ Chaim explained that slavery is more than a denial of wages or a halachic dispensation to marry a shifcha – these are simply consequences of slavery. Slavery is a class status, a social definition that devalues the individual relative to those who are free. It is not enough for eidim zomimim to compensate the accused for lost wages because that does not make amends for the loss of class status they tried to cause.
A married woman needs a get, a document, not just a verbal release from marriage, because marriage is not just a financial obligation, but effects issurim, e.g. she is prevented from marrying other men. In the case of a thief sold into slavery, being a slave permits his marrying a shifcha, and a formal document of release is needed to break that kinyan issur . But why, according to Rashi, does one who sells himself require such a release? He is not permitted to marry a shifcha – the consequence of his enslavement is merely a loss of wages? The answer here too is that the issurim effected in these cases are just symptoms of an underlying status - it is that status which is the cause of a formal release being required and simple mechila not sufficing.
“Ki li bnei yisrael avadim” – it is not the obligations of mitzvos imposed upon us which make us different, but the fact that we are different, that we have the unique status of avdei Hashem, which obligates us to serve Him through those mitzvos.