It leaves Argyle boss Paul Sturrock with a tough decision regarding Mpenza's future.
"We are in discussions to see what the best is for the football club and for Emile," Sturrock told the official club website.
"For services rendered and money we're paying, we haven't really had service from him. It has been a big drain on us financially and it has obviously hampered my thoughts of bringing other people in.
"It is vitally important that we now make a decision. I need a really, really fit Emile, which I have never had or the decision-making would be the other way."