There was a bill introduced earlier this year to allow donations to be accepted by the Department of the Treasury to "plan, design, construct, or maintain a barrier along the U.S.-Mexico international border", but it has gone nowhere. See: H.R.5876 - Border Wall Trust Fund Act - Rep. Black, Diane [R-TN-6] (Introduced 05/18/2018) - This bill authorizes the Department of the Treasury to accept donations to plan, design, construct, or maintain a barrier along the U.S.-Mexico international border. (Latest Action: House - 06/04/2018 Referred to the Subcommittee on Border and Maritime Security).
You don't try to pass a bill that would authorize something that can already be done.