This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate. 

We don't know this to be a fact, but it appears USACE requires a for both inter-agency and intra-agency transactions resulting in contracts.  The FAR does not require but does support USACE's choice to require a D&F.

"FAR 17.502-2 The Economy Act.

      (a) The Economy Act ( 31 U.S.C.1535) authorizes agencies to enter into agreements to obtain supplies or services from another agency. The FAR applies when one agency uses another agency’s contract to obtain supplies or services. If the interagency business transaction does not result in a contract or an order, then the FAR does not apply. The Economy Act also provides authority for placement of orders between major organizational units within an agency; procedures for such intra-agency transactions are addressed in agency regulations.'

"DoD 7000.14-R, Vol 11A Chapter 3 (September 2019)

3.0         LEGAL AUTHORITY (0303)

3.1         Statutory Conditions on Use (030301)

31 U.S.C. § 1535 establishes four basic conditions on the use of Economy Act authorities. In accordance with the statute, the head of an agency or major organizational unit within an agency may place an order with a major organizational unit within the same agency or another agency for goods or services if:

3.1.1.     Funds are available;

3.1.2.     The head of the requesting agency or unit decides the order is in the best interest of the U.S. Government;

3.1.3.     The agency or unit to be asked to fill the order is able to provide or obtain by contract the ordered goods or services; and

3.1.4.     The head of the requesting agency decides that ordered goods or services cannot be provided by contract as conveniently or economically by a commercial enterprise."

We saw nothing specifically related to the topic in DFARS, DFARS PGI, or AFARS.  We think this information supports USACE'S authority to require a D&F.