Donald Trump tweeted the following on August 27, 2017:
With Mexico being one of the highest crime Nations in the world, we must have THE WALL. Mexico will pay for it through reimbursement/other.
According to CNN, Mexico responded as follows:
In response, the country's foreign ministry released a statement saying Mexico would not pay for a wall or other physical barrier at the border "under any circumstances."
"This determination is not part of a Mexican negotiating strategy, but a principle of national sovereignty and dignity," the statement said.
The same article also discusses Trump's suggestion that the US withdraw from NAFTA and Mexico's response:
Mexico said its position at the negotiating table was "serious and constructive" and sought a winning result for Mexico, the US and Canada.
"Mexico will not negotiate NAFTA nor any other aspect of the bilateral relationship through social media or the media," the ministry added.
More recently (December 19, 2018), Trump has claimed that Mexico would be paying for the wall via a new trade treaty:
Mexico is paying (indirectly) for the Wall through the new USMCA, the replacement for NAFTA! Far more money coming to the U.S. Because of the tremendous dangers at the Border, including large scale criminal and drug inflow, the United States Military will build the Wall!
However, both NPR and PolitiFact fact-checked this claim and found it unsupported by the available evidence. I wasn't able to find an official Mexican response to Trump's tweet, but I imagine they would prefer not to reopen negotiations and are deliberately ignoring the President, in keeping with their previous comments quoted above.