There is a way to do this legally. You submit a bill and the organization issues you a cheque for your services. You then write a return cheque back to the organization (the cheques effectively cancel out). This makes it a donation of money and they can then issue you a tax receipt for the donation.
From the horses mouth:
http://www.cra-arc.gc.ca/chrts-gvng/...srvcs-eng.html