Is it permissible for him to take the money left over by the students after a trip because he was not paid anything for supervising?
Question

                I am the manager of classes for Qur’aan memorization. During trips with the students the money is sometimes more than is required for the trip. Is it permissible for me to take this money, knowing that I am not paid anything for my work with these circles? Is it permissible for me to take what I need of this money or to take something from the subscriptions paid by the students to join these circles?.
Praise be to Allah.You do not 
    have the right to take anything from this money, because you have been 
    entrusted to spend it for the purpose for which it was given, which is to 
    take the students on trips. So it is not permissible for you to spend it on 
    anything other than the purpose it was paid for. Allaah says (interpretation 
    of the meaning): 
“Verily, 
    Allaah commands that you should render back the trusts to those, to whom 
    they are due”
[al-Nisa’ 
    4:58]
What you 
    must do is to give the money back to those who paid it, or keep it for them 
    so that you can spend it on the purpose for which it was paid. The fact that 
    you do not receive any salary or remuneration does not allow you to betray 
    the trust and consume wealth unlawfully. You should ask for your right to be 
    given something in return for your work. 
You do not 
    have the right to take anything from the fees paid for the study-circles 
    either, for the reasons stated above. 
What you 
    have to do is to fear Allaah and beware of all haraam wealth. I ask Allaah to help you and guide you and to grant you plentiful provision. 
And Allaah knows best.