CRA indicates that where a GST/HST rebate claimant has underclaimed, it can request an assessment of the claim

Where a public sector body subsequently realizes that it claimed an inadequate rebate respecting a particular transaction for a particular claim period, ETA s. 262(2) prohibits it from filing a second PSB rebate claim in a subsequent claim period for the missed amount in respect of that same matter (i.e., unlike input tax credits, unclaimed rebates cannot be carried forward), and it is also prohibited from filing a second rebate application for the same claim period under ETA s. 259(6). However, CRA states:

CRA could exercise its discretion and reassess the prior rebate claim to add the amount to the PSB rebate claim for the correct claim period.

Alternatively, an applicant can file an objection to the assessment of its initial rebate application requesting that a missed amount be allowed and/or an adjustment be made for a misstated amount.

If the time limit for objecting has passed, the person can write in and ask for a reassessment of the rebate claim for the particular claim period. …

Furthermore, if an auditor is assessing net tax for the claim period in question, an allowable unclaimed rebate amount for that period must be applied against the assessment of net tax.

If this is missed by the auditor when assessing the person, then CRA can apply the eligible unclaimed rebate amount where an objection has been filed by the person.

Neal Armstrong. Summaries of 22 June 2017 Interpretation 180966 under ETA s. 262(2), s. 296(2), s. 296(2.1), s. 261.4(1)(c) and s. 297(4).