https://www.eepcindia.org/circulars/excise/EXC-NN-21-2014.pdf
If Service provider is not paid for service then credit can not be availed [2nd proviso to R 4(7)]
"Provided further that in respect of an input service, where the service recipient is liable to pay a part of service tax and the service provider is liable to pay the remaining part, the CENVAT credit in respect of such input service shall be allowed on or after the day on which payment is made of the value of input service and the service tax paid or payable as indicated in invoice, bill or, as the case may be, challan referred to in rule 9: "
But 3rd proviso says
"Provided also that in case the payment of the value of input service and the service tax paid or payable as indicated in the invoice, bill or, as the case may be, challan referred to in rule 9, except in respect of input service where the whole of the service tax is liable to be paid by the recipient of service, is not made within three months of the date of the invoice, bill or, as the case may be, challan, the manufacturer or the service provider who has taken credit on such input service, shall pay an amount equal to the CENVAT credit availed on such input service and in case the said payment is made, the manufacturer or output service provider, as the case may be, shall be entitled to take the credit of the amount equivalent to the CENVAT credit paid earlier subject to the other provisions of these rules"
Credit can not be availed in 1st place due to 2nd proviso, then why 3rd proviso talking about reversal?