31 U.S.C. § 1531

(a) The balance of an appropriation available and necessary to finance or discharge a function or activity transferred or assigned under law within an executive agency or from one executive agency to another may be transferred to and used— (1) by the organizational unit or agency to which the function or activity was transferred or assigned; and (2) for a purpose for which the appropriation was originally available. (b) The head of the executive agency determines the amount that, with the approval of the President, is necessary to be transferred when the transfer or assignment of the function or activity is within the agency. The President determines the amount necessary to be transferred when the transfer or assignment of the function or activity is from one executive agency to another. (c) A balance transferred under this section is— (1) credited to an applicable existing or new appropriation account; (2) merged with the amount in an account to which the balance is credited; and (3) with the amount with which the balance is merged, accounted for as one amount. (d) New appropriation accounts may be established to carry out subsection (c)(1) of this section.