CRA confirms that there is no requirement on a supplier to refund GST/HST on an excess charge
ETA s. 232(2) provides that where a supplier has charged GST/HST on consideration that is subsequently reduced, the supplier “may” then refund the GST/HST it charged on the excess consideration. CRA stated:
Note that the legislation uses the word “may”. As such, it is at the discretion of the [supplier] whether to refund the GST on the reduction in consideration.
Neal Armstrong. Summary of 21 December 2017 Ruling 167830 under ETA s. 232(2).