48 C.F.R. § 52.232-11
52.232-11 Extras.
As prescribed in 32.111(c)(2), insert the following clause, appropriately modified with respect to payment due dates in accordance with agency regulations, in solicitations and contracts when a fixed-price supply contract, fixed-price service contract, or transportation contract is contemplated:
Except as otherwise provided in this contract, no payment for extras shall be made unless such extras and the price therefor have been authorized in writing by the Contracting Officer.