No part of any appropriation disbursed by the Secretary of the Senate shall be available on and after July 12, 1960, for the payment to any person, at the time of the service upon him of a subpena requiring his attendance at any inquiry or hearing conducted by any committee of the Congress or of the Senate or any subcommittee of any such committee, of any witness fee or any sum of money as an advance payment of any travel or subsistence expense which may be incurred by such person in responding to that subpena.