Unpaid
Part of speech: adjective
Definition: Not paid for.
Example sentence: Although attracted by the humanities, I had chosen medicine as a career, seduced by the image of the 'man in white' dispensing care and solace to the suffering. But science was lurking around the corner, in the form of an unpaid student assistantship in the laboratory of physiology.
Voluntary
Part of speech: adjective
Definition: Done, given, or acting of one's own free will.Working or done without payment.
Example sentence: Truly it is an evil to be full of faults; but it is a still greater evil to be full of them and to be unwilling to recognize them, since that is to add the further fault of a voluntary illusion.