Canadian registered charities cannot issue official donation receipts for services such as a volunteer spending time or a donation of office space.  Here is more information from the Charities Directorate of CRA:

“Gifts of services

Can a registered charity issue official donation receipts for gifts of services?

A charity cannot issue a receipt for a gift of service. At law, a gift is a voluntary transfer of property without consideration. Contributions of services (for example, time, skills, effort) are not property. Therefore, they do not qualify as gifts for the purpose of issuing official donation receipts.

Registered charities cannot issue official donation receipts for gifts of services. However, they can issue receipts under the following conditions:
• If a charity pays a service provider for services rendered and the service provider then chooses to donate the money back, the charity can issue a receipt for the monetary donation (this is often referred to as a cheque exchange). In such circumstances, two distinct transactions must take place:
o a person provides a service to a charity and is paid for that service; and
o that same person makes a voluntary gift of property to the charity.
• A charity should also make sure that it keeps a copy of the invoice issued by the service provider. The invoice and cheque exchange not only ensure that the charity is receipting a gift of property, but they also create an audit trail, as the donor must account for the taxable income that is realized either as remuneration or as business income.
• A charity should not issue an official donation receipt to a service provider in exchange for an invoice marked “paid”. This procedure raises questions as to whether in fact any payment has been transferred from the charity to the service-provider and, in turn, whether any payment has been transferred back to the charity.
• CSP-S03, Services
• CPC-017, Gifts of Services”