I believe that David posted the correct answer to your question.
I believe your confusion comes as the result of not understanding the terms contract and slavery correctly.
A slave, by definition, is physically forced to work for another individual against his own will. Thus, agreeing to be a slave does not make any sense, it is a contradiction in terms. One can not agree to be a slave while being physically forced to work for another individual against their own will.
A contract, by definition, is a legally protected agreement between two or more individuals in which all of the participants voluntarily choose to enter in said agreement. As I already stated, a slave is a person who is physical forced to work for another individual, against their own will. As such, it is a contradiction to say that an individual can “agree to be a slave.”
You can agree to provide certain services to another individual, without payment on their part, but such an agreement can not be referred to as “agreeing to be a slave,” but rather it is a contractual agreement.
An individual has the right to engage in such a contractual agreement, but the individual at the end of that agreement DOES NOT have the right to turn such an agreement into slavery, which is, by definition, forcing one individual to work for another against their own will.
I believe that this is what Mr. Binswanger meant when he said that no one has the right to be a slave (such a concept makes absolutely no sense since it is a contradiction).
If you could provide us with an actual transcript of the quote in which he stated this, or at least some of the context behind this statement, perhaps we would be better able to answer your question.