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 However, later it was withdrawn.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!
This opinion is based upon the information provided by you. You are providing the services for recruitment of candidate required by your client as per predetermined terms and condtions.
The last assignment done by you no way affected by the termination of agreement. The action of the client would affect the future/new assignment but not the selection process done as per the direction of the client.
Your client is bound to pay for the last assignment done by you prior to termination of contract. The client is legally bound to pay because your action is as per their direction.
You raise the invoice quoting their reference of conducting selection. If your client does not pay then take the leagal course for the same.