http://decisions.fca-caf.gc.ca/fca-caf/decisions/en/item/71287/index.do 
Canada v. Quinco Financial Inc. (April 29, 2014 – 2014 FCA 108) was an appeal from a decision of the Tax Court which essentially concluded that there was a legislative gap in the rules dealing with the GST consequences of debit/credit notes:
http://decision.tcc-cci.gc.ca/tcc-cci/decisions/en/item/31068/index.do 
A predecessor corporation of the taxpayer had claimed ITCs net of credit notes and debit notes issued by or to suppliers, i.e., the GST claimed was based on the adjusted purchase price of supplies made to it rather than the original price. Subsequently, but within the statutory period for ITC claims, the predecessor had claimed additional ITCs representing the ITC reductions arising from credit notes and debit notes (at the date of trial, the amount in dispute was approximately $3.1 million), i.e., based on the unadjusted original purchase price. The taxpayer argued that the relevant provision of the
Excise Tax Act, paragraph 232(3)(c), only limits ITCs for periods during which, or prior to which, the credit notes or debit notes were received or issued, but did not limit claims made in periods after such receipt or issuance:
(3) Where a particular person adjusts, refunds or credits an amount in favour of, or to, another person in accordance with subsection (1) or (2), the following rules apply:
. . .
(c) the amount shall be added in determining the net tax of the other person for the reporting period of the other person in which the debit note is issued to the particular person or the credit note is received by the other person,
to the extent that the amount has been included in determining an input tax credit claimed by the other person in a return filed for the reporting period or a preceding reporting period of the other person; and
[Emphasis added]
The Tax Court agreed with the taxpayer and allowed the appeal with costs:
[27] In this appeal, paragraph 232(3)(c) does not apply. It was established in evidence that with respect to the deferral periods, the appellant did not claim the ITCs in the same, or a preceding, reporting period as the periods in which the credit or debit notes were issued or received. No addition to net tax is mandated where an ITC was claimed after the reporting period in which a debit or credit note was issued or received.
[28] The respondent also argued that the appellant’s interpretation of subsections 168(1), and 169(1) and paragraph 232(3)(c) does not respect the object and spirit of the Act, which is to attain tax neutrality as between GST paid to suppliers in the course of a registrant’s commercial activity and ITCs claimed.
[29] As I stated earlier, subsection 232(3) of the Act is, in my view, the provision applicable when credit and debit notes are received or issued.
[30] The Supreme Court of Canada stated in the decision of
Canada Trustco Mortgage Co. v. Canada, [2005] 2 S.C.R. 601, that when the words of a provision are clear and unambiguous the words should be given a dominant role in the interpretative process, particularly when dealing with provisions that specify precisely what conditions must be satisfied to achieve a particular result. …
The Federal court of appeal dismissed the Crown’s appeal:
[2] We have not been persuaded by the Crown’s arguments, all of which involve a strained interpretation of the provisions of the
Excise Tax Act, R.S.C. 1985, c. E-15 so as to avoid a result that the Crown, for obvious reasons, regards as a windfall.
[3] This case falls to be decided by the usual principles that govern the interpretation of provisions in taxation statutes. The Supreme Court has set out the governing principles in
Canada Trustco Mortgage Co. v. Canada, 2005 SCC 54, [2005] 2 S.C.R. 601.
…
[9] There may be cases where precisely-worded provisions or their interaction creates an advantage or a windfall for a registrant under the Act. But we do not interpret taxation provisions in a tendentious or result-oriented way to enhance the federal treasury:
Shell Canada, supra at paragraphs 39 and 40. Instead, absent words allowing us to address situations of abuse or windfall, where the provisions are precisely-worded, clear and unambiguous, they must be given their plain effect.
[10] In this case, we agree with D’Auray J.’s interpretation of the combined effect of subsections 169(1), 232(2) and paragraph 232(3)(c) of the Act.
[11] As a result, the appeal will be dismissed with costs.
Note: On January 17, 2014 the Department of Finance introduced a proposed amendment to paragraph 232(3)(c) of the Excise Tax Act eliminating this “gap” retroactively to April 23, 1996:
http://www.fin.gc.ca/drleg-apl/2014/eta-lta-0114-l-eng.asp
According to a Department of Finance press release issued the same day:
The proposed amendments will generally apply as of the day the provisions being amended originally came into force. However, the proposed amendments will not affect any case that has already been decided by the courts.
http://www.fin.gc.ca/n14/14-006-eng.asp
Thus absent an appeal to, and reversal by, the Supreme Court of Canada it would appear that the taxpayer will be able to retain the proceeds of its successful appeal.