“the courts have been clear that uncertainty about whether Congress has authorized eminent domain has to be resolved in favor of the property owner”
There is no such uncertainty.
Congress has passed laws that grant the Executive Branch broad authority to determine what is in the public interest, since the condemnation of 1888. Powers were expanded under the New Deal, and specific authority for taking border lands has been written into several laws.
It was widely done under the 2006 Secure Fence Act (the Bush-era barrier program).
No doubt there are many laws and expanded laws, but since you looked that up how about a link like I provided.
Different circumstance if Congress authorized funding.