5. Payment of tax in case of new services.- Where a service 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 fourteen days of the date when the service is taxed for the first time.