Dear Rachna,
You have provided your client the recruitment services. As a part of that, the selection process completed and the client had issued the offer letter. However, later it was withdrawn. Notwithstanding the withdrawal of the offer letter, I recommend you raising the invoice for the services. If the client raises the objection then tell them that the time and energy that you have spent in searching the candidates deserves payment. Withdrawal of offer was because of their internal reasons and you as a service provider has nothing to do with it.
While you have raised query about payment for your services, what about the candidate? Sudden withdrawal of the offer letter must have left him high and dry. Where he will go? Now he will be jobless till he gets a new job. Moreover, a status of being jobless weakens his candidature. He has lost his bargaining power. His worries are far more severe than yours!
Thanks,
Dinesh Divekar