This is exactly one of the situations the 25th Amendment was designed to handle:
Donald J. Trump: "We have fully signed and documented another very important part of the Immigration and Security deal with Mexico, one that the U.S. has been asking about getting for many years. It will be revealed in the not too distant future and will need a vote by Mexico’s Legislative body!." David Rothschild: "Take a step back: [the] US President is repeatedly, publicly [claiming] that he made a massive international deal with Mexico that everyone in both the Mexican government and our government says does not exist[;] that is (1) disturbing behavior [that is] (2) incredibly damaging [to] US security/economy." Nicole Wallace: Trump appears less capable of sustaining his own thoughts for longer than 4 to 6 seconds. It used to be he couldn’t follow anyone else’s conversation but now it seems he can’t follow his own train of thought...
#noted