, That the Working Capital Fund will be fully reimbursed by each customer agency from available funds for the actual cost of the transit benefit. 104. None of the funds in this Act may be obligated or expended for retention or senior executive bonuses for an employee of the Department of Transportation without the prior written approval of the Assistant Secretary for Administration. Federal aviation administration Operations (airport and airway trust fund) For necessary expenses of the Federal Aviation Administration, not otherwise provided for, including operations and research activities related to commercial space transportation, administrative expenses for research and development, establishment of air navigation facilities, the operation (including leasing) and maintenance of aircraft, subsidizing the cost of aeronautical charts and maps sold to the public, the lease or purchase of passenger motor vehicles for replacement only, in addition to amounts made available by Public Law 115–254 , $10,540,511,000, to remain available until September 30, 2021, of which $10,540,511,000 shall be derived from the Airport and Airway Trust Fund: Provided , That of the sums appropriated under this heading— (1) $1,359,607,000 shall be available for aviation safety activities