Dear Sayali,
The recruitment agency is a contractor in this case. The contractor is eligible to raise the invoice provided the candidates are selected. If he provides the candidates but none of them is selected, then the contract services are incomplete. For the incomplete service, how the contractor will be eligible to raise his invoice?
Anyway, in your contract, you may write clearly that:
"Selection of the candidates is subject to the fulfilment of the recruitment standards of _______ (name of your company). There is a possibility that none of the candidates recommended by the contractor could get selected. Under such circumstances, the contract services will be deemed incomplete and the contractor will not be eligible to raise their invoice."
Before you avail of the services, you may issue a proper Purchase Order (PO) or the Work Order (WO) to the recruitment service provider. Mention clearly the terms and conditions of the services, the general process of selection, terms of payment etc. At the same time, give a commitment to the recruitment service provider that the candidature whose candidature is turned down will not be selected in the same position or any other position for the next five years.
Thanks,
Dinesh Divekar