Huh? That is in fact the normal state of indentured servitude. Slavery is also commonly a voluntary situation; that's the whole concept of "selling yourself into slavery".

> Until the late 18th century, indentured servitude was common in British America. It was often a way for Europeans to migrate to the American colonies: they signed an indenture in return for a costly passage.

( https://en.wikipedia.org/wiki/Indentured_servitude )