The federal anti assignment statutes prohibit an attorney from asserting a claim on the basis of a contract that creates a lien upon or a property interest in the proceeds of the claim against the United States.  A provision in a contract for the prosecution of a claim against the United States, which purports to make the contingent attorney’s fee therein provided for a lien upon any warrant, which may be issued in payment of such claim is repugnant to U. S. Rev. Stat. 3477 (annulling assignments of such claims).[i] However, an agreement between a client and an attorney to pay the attorney a certain fee from amounts to be recovered in prosecuting the client’s claim against the United States does not fall within the prohibition of the Assignment of Claims Act.

Pursuant to the Omnibus Claims Act of March 4, 1915, it is unlawful for any attorney to exact, collect, withhold, or receive any sum which, in the aggregate, exceeds 20 per cent of the amount of any item appropriated in that act, on account of services rendered or advances made in connection with a claim, any contract to the contrary notwithstanding.

[i] Calhoun v. Massie, 253 U.S. 170 (U.S. 1920)