Really? Last I checked, the US provides Mexico with a significant amount of foreign aid. That could always be diverted to build a wall instead.
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So the American taxpayers, from whom that aid is collected, will be paying for the wall.
That aid is not Mexico’s money, it is our money.
Besides there are plenty of other measures that can be enacted to assert more pain on Mexico's economy, such as making it illegal for any private monies to flow into Mexico, which would be disastrous to Mexico's economy.