Question: In 2016, Trump's administration [issued a plan](https://assets.donaldjtrump.com/Pay_for_the_Wall.pdf) on how to force Mexico to pay for the wall:

> 
> * On day 1 promulgate a "proposed rule" (regulation) amending 31
>  CFR 130.121 to redefine applicable financial institutions to include
>  money transfer companies like Western Union, and redefine "account" to
>  include wire transfers. Also include in the proposed rule a requirement that no alien may wire money outside of the United States unless the alien
>  first provides a document establishing his lawful presence in the United
>  States.
> * On day 2 Mexico will immediately protest. They receive approximately $24
>  billion a year in remittances from Mexican nationals working in the United
>  States. The majority of that amount comes from illegal aliens. It serves as de
>  facto welfare for poor families in Mexico. There is no significant social safety
>  net provided by the state in Mexico.
> * On day 3 tell Mexico that if the Mexican government will contribute the funds
>  needed to the United States to pay for the wall, the Trump Administration will
>  not promulgate the final rule, and the regulation will not go into effect.
> 
> 
> 

Further sections proposed enacting trade tariffs and increasing visa fees, which also didn't seem to have happened. Did President Trump ever explain why the original plan was abandoned?

Answer: 
Because it turns out it was all bluster. According to the [Washington Post](https://www.washingtonpost.com/graphics/2017/politics/australia-mexico-transcripts/?utm_term=.a7dd3039482a), 
the timeline is as follows:

> 
> **1/20/17**: Trump inaugurated.
> 
> 
> **1/25/17**: Trump signs an executive order to begin construction of a southern border wall. Jared Kushner meets with Mexican Foreign
>  Minister Luis Videgaray to craft the language for the announcement,
>  which is “well received” in Mexico and **doesn’t discuss payment**.
> 
> 
> **1/26/17**: Trump tweets, once again, that Mexico will pay for the wall. Facing political pressure at home, (Mexican President) Peña
>  Nieto cancels his Jan. 31 trip to Washington.
> 
> 
> **1/27/17**: Trump calls Peña Nieto in response to the cancelled trip. In it, Trump starts out by threatening Mexico with tariffs, and brags
>  about how many hispanics voted for him. Peña Nieto acts surprised, and
>  mentions that a tariff was not what their people had been discussing.
>  Trump (falsely) says it's what he's been telling his (huge) rallies he
>  was going to do a tariff for forever. 
> 
> 
> Peña Nieto counter-offers to work with Canada to update NAFTA if
>  tariffs is Trump's desire. Trump pivots three times in his response,
>  starting with complaining about Mexican drugs, pleading that he has to
>  have Mexico pay for this wall, then saying how cheaply he can build
>  it. 
> 
> 
> Peña Nieto mentions that money and guns coming from the USA are also
>  helping fuel the drug trade, and suggests that all parties just stop
>  discussing the wall. They agree, and Trump says he wants Peña Nieto so
>  popular in Mexico that they remove term limits for him. They agree to
>  have Jared and Luis draft a positive statement that doesn't mention a
>  wall, and hang up.
> 
> 
> 

So, bottom line? Because Peña Nieto told Trump 'no', and Trump would rather stop all discussion on the wall than have Peña Nieto continue to tell the press that he wouldn't be paying for it.
No, Trump nor the administration has not explained why. But this plan has been analysed in the media and found to be practically difficult to implement.

Firstly there are no relevant regulations at 31 CFR 130.121, The numbering system changed many years ago. Probably Trump was referring to 31 CFR1020.220 (which used to be numbered as CFR 103). This suggests that the details of the policy had not been worked out when it was first suggested.

The plan involves redefining Western Union as an applicable financial institution. However, Western Union and other money transfer companies are already defined as financial institutions in these regulations.

The proposal could be rewritten to only allow US citizens and documented aliens to open Western Union (or similar) accounts. This would prevent a wide range of people from using money transfer. For example, a European businessman could not open an account with WU in the US to transfer money from his interests in the US back to Europe as the European businessman is not legally resident in the US. 

And you can't differentiate between a European businessman, a Mexican Businessman (in Mexico City) and a Mexican agricultural worker. Either they are all allowed, with proper identification, to open a WU account, or none of them are.

(Summarised from <https://www.steptoeinternationalcomplianceblog.com/2016/04/trumps-mexico-remittance-proposal-goes-inexplicably-awry/>)