1)The word used is "revised tax invoice" in rule 53.
2) In law, the word "may" is often read as shall when there is something in the nature of thing to be done which makes it the duty of the person on whom the power is conferred to exercise the power. Yes the word "may" has been used but as per my opinion if rule only "directory" in nature then it will only create confusion as to who will issue revised tax invoice and who will not and thus this rule is to be interpreted as "mandatory" in nature.
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