Trump likes to say you have to be willing to walk away from a deal.
ADDED: The next presidential election in Mexico is in 2018, and the incumbent is not eligible to run. So Nieto has no election to worry about. According to Wikipedia:
Peña Nieto began his term as President with an approval rating of approximately 50%, but the falling value of the Mexican Peso and sluggish economic growth had caused this figure to drop to 35% by 2015. Prone to gaffes, Nieto has also faced several major controversies as President.... According to the Newspaper Reforma, by August 2016, Peña Nieto's approval ratings had dropped to 23% - the lowest for any president since the paper began opinion polling in 1995. The approval decreased more by January 19, 2017, remaining at 12%.He won the election in 2012 with 38% of the vote.
ADDED: What would Trump do if he — with his art-of-the-deal genius — were in Nieto's position?