Credit note
A document that reduces the value of an invoice already issued — for a return, a later discount or an overcharge.
Once an invoice is issued and reported, you do not edit it. If goods come back, a discount is agreed after the sale or you charged too much, you issue a credit note against the original invoice, and the GST on it comes down with it. A debit note does the opposite, for an undercharge.
Both are reported in your GST returns, so they need the same care as the invoice they correct.
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See also: Tax invoice