Waiver of government revenues: cancellation of wrongly issued recovery certificates is not waiver, authorities may amend notices under procedural rule. Cancellation or non-enforcement of a wrongly issued recovery certificate is not a waiver or abandonment of government revenues where no amount is due; Tax Recovery Offices may amend earlier notices by invoking rule 87 of the Second Schedule to address costs of execution, with the Board endorsing this procedure for cases falling within the prescribed small-cost threshold.