Where a service, not being a service covered by rule 6, is taxed for the first time, then,

(a) no tax shall be payable to the extent the invoice has been issued and the payment received against such invoice before such service became taxable;

(b) no tax shall be payable if the payment has been received before the service becomes taxable and invoice has been issued within the period referred to in rule 4A of the Service Tax Rules, 1994.