2 U.S. Code § 68–8 - Vouchering Senate office charges
(a) Senate support office charges
Charges for expenses of any office, the funds of which are disbursed by the Secretary of the Senate, may be vouchered by a Senate support office paying such expenses or to which such charges are owed for goods or services provided, if—
(1) such charges are paid on behalf of the office incurring such expenses by such Senate support office; or
(b) Payment charged to official funds
Payments under this section shall be charged to the official funds of the office on whose behalf the expenses were paid, or which received the goods or services for which payment is required.
Any voucher submitted by a Senate support office pursuant to this section shall be accompanied by a certification from such office of the amount and that such purchases were of the nature that they could be charged to the official funds of the office on whose behalf charges were paid, or to which goods or services were provided.