Gilles: You cannot give a charitable receipt in Canada for labour performed. You can only give a receipt for the transfer of property. If the person doing the work provided any goods of value, ie paint, hardware, etc, you can give a receipt for those. But labour is not something you can give a receipt for. This has been ruled on by the courts, and no chartered accountant would touch such a receipt with a 10 foot pole. You would be putting your organization's charitable/gifts to the crown tax status at risk.
For a bit of a read on the subject, see here:
http://www.cga-canada.org/en-ca/Abou..._taxforum.aspx