GST/HST Memorandum 12.2 [aka 12-2] "Refund, Adjustment, or Credit of the GST/HST under Section 232 of the Excise Tax Act" April 2008

Reduction in consideration must be given to original recipient re amounts already charged and must not relate to any action of the recipient

6. A reduction in consideration may occur under the following circumstances:

• when some or all of the consideration is returned;
• as a result of surpassing a certain volume of purchases, i.e., a volume rebate;
• where goods delivered are found to be substandard; or
• where goods are returned to the supplier for a full or partial refund of the consideration.

7. The reduction in consideration must be given to the original recipient of the supply, or that person's agent, and must relate to an amount that has been collected or charged.

8. The reduction in consideration must also relate to the original supply and may be made for any reason but must not depend on any action undertaken by the recipient or any supply made by the recipient. ...

12. There is no requirement for the supplier to refund, adjust, or credit the GST/HST charged or collected....

20. ...If the refund, adjustment, or credit of the tax relates to more than one invoice, the note should indicate the dates of the first and last invoices issued. ...

22. There is no requirement to issue a credit note or debit note unless a refund, adjustment or credit of the tax is made by the supplier to the recipient. However, no corresponding adjustment to net tax is permitted unless a credit note or debit is issued.