CRA considers that the GST manufacturer’s rebate rule is only available for rebates paid to a third party

Although on its face, ETA s. 181.1 could apply where a registered vendor refunds part of the selling price together with GST to a registered purchaser, CRA’s position is that s. 181.1 only "applies to rebates paid by a supplier to third parties with whom the supplier was not dealing directly."  The significance of this is that if the vendor does not issue (or receive) a credit note (or debit note) in the proper form for the adjustment, so that the vendor is not entitled to an input tax credit for the refunded GST under s. 232(3), the vendor also will not be entitled to claim an ITC under s. 181.1.  However, in this situation CRA considers that the purchaser nonetheless is required to remit the refunded GST on general principles even though there is no explicit addition to its net tax obligation under s. 232.

Neal Armstrong.  Summary of 8 July 2013 Interpretation 145134 under ETA s. 232(3).