be credited to this appropriation as offsetting collections and shall be available only for aviation security: Provided further , That the sum appropriated under this heading from the general fund shall be reduced on a dollar-for-dollar basis as such offsetting collections are received during fiscal year 2021 so as to result in a final fiscal year appropriation from the general fund estimated at not more than $4,987,407,000. Procurement, construction, and improvements For necessary expenses of the Transportation Security Administration for procurement, construction, and improvements, $154,492,000, to remain available until September 30, 2023. Research and development For necessary expenses of the Transportation Security Administration for research and development, $29,524,000, to remain available until September 30, 2022. Coast Guard operations and support For necessary expenses of the Coast Guard for operations and support including the Coast Guard Reserve