Whenever there is no Vice President, the President of the Senate for the time being is entitled to the compensation provided by law for the Vice President.
Structure US Code
SUBCHAPTER II— VICE PRESIDENT AND PRESIDENT PRO TEMPORE
§ 6111. Compensation of President pro tempore of Senate
§ 6112. Compensation of Deputy President pro tempore of Senate
§ 6113. Appointment and compensation of employees by President pro tempore of Senate
§ 6115. Expense allowance of President pro tempore of Senate; methods of payment; taxability
§ 6116. Special delivery postage allowance for President of Senate